Treating the statements of an author as canonical when they speak or write about their own work is a venerable practice that long predates contemporary works created for entertainment value. Referring to the practice as the "Word of God" comes from its origins in the interpretation of sacred writings that entire faiths believe were inspired by or transcribed from the teachings or utterances of a supreme being. A clue to this can be found in the definitions of canon which include "a Church decree or law" and "a collection or list of sacred books accepted as genuine". Is it preposterous to apply the same kind of validation to a script of "Buffy the Vampire Slayer" as you would to a book of The Holy Bible? Or is going to the source when in doubt merely a cultural consistency?
Were holy texts actually the literal word of a god, the only way to know what they truly meant would be to ask the Creator yourself, and heaven knows plenty have made claims of doing so and getting an answer. Thing is, they're not. They were written by human beings, usually centuries ago, often in some now-dead language, and then translated to another language by other human beings, then translated again by yet another... each time having their original meaning slightly diluted, modified, obscured, or otherwise corrupted from the original text (Your Faith May Vary™). In many cases, the changes weren't simply artifacts of translation, but intentional modifications made for the purposes of social control and protecting the power and authority of a particular church, or sect, or group of priests, or a king, or whatever.
So when scholars studying these words arrive upon some passage that is inscrutable or vague, and the questions of "what the FOLK does this even mean?" arise, as they always do, it becomes more than simply a matter of curiosity as to whether the author meant for the characters to fancy one another in a particular way. It can become a rather more desperate matter of affirming and validating specific tenets of an entire cult around which, in their view, the entire human race must adhere or face eternal damnation. In such situations, scholars study earlier translations and any other historical evidence available for clues, hoping to ascertain the true and seemingly hidden meanings. Some people spend their entire lives doing this, a preoccupation dating back centuries, while others spend their lives trying to stop them from doing it. Rather more obsessive than fandom in my view.
Hang on, there's more. Did you know that this sort of scholarly pursuit of truth "outside the text" is also common in legal contexts? To take one example, let's consider The Constitution of the United States of America and the Amendments to the Constitution of the United States of America. You've heard of The Federalist Papers, right? It's some quaint little thing the founding fathers used to expound upon their ideas and intentions, stuff that just didn't get into any of the drafts that were written of the Constitution. There's all sorts of other writings they left behind too, some even conflicting with each other — what fun! And today there are thousands of people in the United States who in complete seriousness put on black robes and sit on high benches to pass judgment about such matters — nine of them in particular have the last word when it comes to answering "WTF?" questions about what's written inside the text of this document. In fact, there's a hot trend among the current gang of nine to actually ignore the decided precedent established by hundreds of years of decisions made by earlier members of their gang, and instead imagine what the original authors of the texts in question actually meant by cherry-picking primarily from things they wrote outside the text, or even things the authors allegedly read (!?) that were written by other people. If you think that takes big balls, sometimes they'll whip out some new legal theory that was cooked up by partisan hacks who are bribing them to rule in their favor on bullshit cases whipped up in state courts for the express purpose of overturning existing federal case law. But, uh, I digress.
These are just two of the cultural drivers behind the tendency within entertainment fandom to seek knowledge that can't be clearly derived from the text. I'm not seeking to argue that it's good or ideal, I'm just making the point that it's not surprising or even the slightest bit unusual. There are many more examples which relate to other kinds of historical scholarship on literature, both for fiction and non-fiction. This pursuit has been going on ever since the first person started writing things down, and it's never going to stop because no matter how much you struggle to make yourself clearly understood in writing, the readers just can't climb inside your head and see your vision perfectly the way you do. Indeed, your inability to articulate that vision the way you intended may not be because the suits didn't like it, or the studio forced you to cut 20 minutes because it was "too long", or it would face conservative backlash, or upset the fan base, or (insert other valid reasons here), but due to it not being shot full of holes in front of you before you published it. No matter how hard you try to think of everything, you almost never do. In such cases it becomes paramount that you add to the incomplete or poorly-expressed stuff you already put in writing to prevent your words from being misinterpreted by some smartass who then pronounces to the world that since your words made their brains go "thud", you're obviously liberal, gay, socialist, racist, ableist, homophobic, xenophobic, misogynistic, misanthropic, or some other myopic pejorative. Or, in less abusive terms, they have some agenda to push like heteronormativity and they're using your words to drive it when that ain't what you had in mind.
The truth is unusually complicated these days because a surprisingly large minority of folks refuse to accept any reality that disagrees with their worldview. Telling it like it is becomes challenging when there are other people who want that story to be received and understood through their personal lens. Going to the source outside the text is a way of cutting these people off from their mission drive to hijack the message and make it serve their ends. Sometimes those ends are good, and sometimes they're evil. When the source is disputed and no authority exists or is recognized as a representative of the source, that's when things can get really unpleasant. Many wars throughout history have been fought over this issue, and countless hordes have died asserting or defending one point of view over another. Who's right? Only the winner decides how history is written. Unless you're, like, Howard Zinn or something.