Hmmmm, I don't remember it popping open. I do think you can press on the film door and it moves in and out a bit. Maybe it doesn't close properly/fully.
It might not have been you that popped the back open... (could have been, ohhhh, let's say the kitty-cat...)...OR as you mentioned, it could be that the light seals are not quite up to 100% par, and if the back moves in and out, that might be just enough to allow a few hundred thousand tiny little
sneaky photons to
sneak past the foam barrier wall between the two jurisdictions (Outside, and Inside).