What would cause a snake to trow up?

An intestinal blockage, parasite, a 'cold', or "Fear" can all cause them to regurgitate their food.

If it was NOT digested, or partially digested then it is most likely that you handled the snake too soon after eating, or caused it to fear (from some action). While not good it is far less life threatening. Clean up the mess, leave it alone for a week or two (dependent on age and need of food) and then try feeding it again.

If it was digested, or partially digested then it can be a lot more serious and you need to see a vet (that specializes, or KNOWS about snakes) - your avg corner vet can't get the job done - make sure they are a Herpetologist.