According to the person who told me about it, L4D1/2 calls the incap event so that the death animation can play otherwise the tank would just ragdoll or require much more complicated coding to use the death animation.
Also whatever you do, don't give the tank health while he is in his death animation or it will loop continuously with his voice getting angrier and angrier.
offtopic but glad to know this info because i used the weapon unlock plugin and made awp's do 99999 damge and wondered why tanks never did the death animation