If you stun a boss while he is in the animation of doing an attack, he will use a different skill on top of the already ongoing attack.
E.g. on the Rayeh fight, if he does the “slices” attack where he stands in the middle, if you tank through it an keep stunning him he will use different abilites like the big slam or the dash while the slices keep going. I think this goes for all bosses but its especially easy to notice on Rayeh.