Aerodynamics.
When an airplane goes into a roll, the direction and speed of that roll is controlled by deflection of the ailerons. Ordinarily, a downward aileron deflection increases lift. When the right aileron is down, the plane rolls to the left, and vice versa.
However, because real wings are not perfectly rigid, the deflection of ailerons causes the wing to twist, which changes the angle of attack. The leading edge will twist downward, creating a decrease in lift.
At low speeds, this effect is fairly small, and a downward deflection will still increase the lift and the plane will roll in the expected direction.
However, when the plane reaches "reversal speed", the combined deflection of the aileron and the wing twist creates a net lift of zero - the plane doesn't roll at all! Above reversal speed, it gets worse - the net lift from an aileron deflected downward is negative! The plane will roll right when you expect it to roll left, and vice versa.
Pilots of high speed aircraft need to be aware of the reversal phenomenon, so that they can accurately compensate by doing the reverse of what feels right. This can be very confusing, and has resulted in more than a few wrong turns. These days, though, control systems can be designed to compensate automatically for the effects of reaching reversal speed. | 
