It’s set up as a surprise based on the fact that Annie caught fire earlier in the film when she attempted to burn the book, but it’s never really explained why Steve is the one who ignites at that moment — after all, he had nothing to do with the burning, and he had every intention of leaving.
Why Did Steve Burn?
One of Hereditary’s scariest moments (and one that was sadly teased in the film’s creepy trailer) is the scene in which Annie (Toni Collette) accidentally burns Steve by throwing Charlie’s book into the fire.
There’s a case to be made that Annie merely is behind the change and was lying when she feigned shock, but Hereditary definitely keeps us in the dark as to why this actually happened the way that it did.