I don't think pizza is a trigger food for this person, but it could be. Funny though - we ended up with only 2 pieces left, so there really wasn't enough to pig out on.
Nah, but it could've meant her being unable to keep from picking up a large Supreme or two on her way home.
Besides that, I can think of a lot of things that might play in.
- She might have medical restrictions, even temporary ones, she is not comfortable talking about, even to the host. (Especially if anyone who she might tell is a known gossip--not casting aspersions, *I* don't know who was at your party.
- She might have OCD, where she doesn't feel comfortable eating food that's been prepared by someone else.
- She might really, really, hate pizza.
- She might have allergies to something on pizza and be too shy to admit to it, lest she feel like she's forcing the hostess to change the menu--or fix additional food--on her behalf, so she brought her own food.
- She didn't want to add to the cost of pizzaing everyone.
- She could have misunderstood and thought it was a "We're doing salad and pizza, bring whatever else you want" meal. My friends and I used to do those time to time (and one resulted in my own Etiquette Hell story I should share one day).
- She could be struggling with her diet/willpower, and need to stay as on track as she possibly can, lest she goes off the rails entirely.
In short, there's a lot of reasons. If this has been a fancier dinner, one that wasn't casual, there'd be a greater risk of an etiquette fail. But it was casual, she cleared it with the hostess, *and* by bringing her own ready-made food, she made sure she didn't inconvenience said hostess. I'd say it was odd, but that she's in the clear on this one.