My gaming memories go way back to the Atari 2600 with its single-button gamepad, and with computer gaming I started with the PC XT.
Back in the day, movement would be controlled by the arrows and actions with the spacebar (and secondary actions with Ctrl, Alt, Shift, etc). That was the standard for a long time; I remember playing games on my Pentium with these key bindings.
Nowadays, WASD is almost universal, and even flash games on popular gaming sites accept these keys as movement input.
What was responsible for this change? Can it be traced back to a popular game, important event, or trend that introduced and shifted the tendency from the former to the latter? Or was it just a natural process? If so, why did everyone choose the same keys?