I can’t answer in detail, but as I recall the allegation from the film he ‘panicked’ and blew the hatch by whatever means were provided for that purpose. IRL he did not have the bruise that would certainly have shown up if he did panic and deliberately blow the hatch. I don’t recall that being mentioned in the film. It’s been a while since I’ve seen it, but I think the editing implied that maybe his helmet ‘hit the switch’ (after he seemed to become cluastrophobic in the helmet); so maybe he panicked getting the helmet off and the helmet ‘hit the switch’.
In any case, my recollections are that Grissom did not panic or do anything wrong, and the hatch really did ‘just blow’. Engineers discovered that a hatch could blow inadvertently, and did not install one on Apollo I. Ironically, the astronaut whose ship was lost after the hatch was lost was killed because his new spacecraft did not have an explosive hatch.