It happens if you have an ability keybound (with modifiers) to the same thing you use for movement, usually WASD. If you bind Ctrl/Alt/Shit + W, A, S or D, you'll stop moving as soon as you hit one of the modifier keys because the game is thinking that you're trying to press, for example, Ctrl+W instead of just W to move forward.

Other games deal with this by not activating Ctrl+W until you let off the W key but this game does not. In the meantime I just don't use WASD for any ability binds.