Well since the time travel was "Paradox Free", it reordered things to happen. Seymore did not always petrify that way. But since he needed to petrify so the future did not change, he did, even if it was in a different manner.
See, that's the thing, I don't remember him actually petrifying before. That episode doesn't end him with him petrified - it ends him him laying down on th sidewalk, still waiting for Fry. Since it ended that way, I wondered how he ended up petrified in the standing position.
But then the movie came along and made it make sense!
