After inadvertently activating a portal gun and bringing Chell to Equestria, Derpy found herself imprisoned in the Crystal Caverns. She knows that Princess Celestia will eventually relent and let her out; Derpy just has to be patient.
Sparkler isn't patient. And Sparkler has some very particular skills.
Ponies shouldn't play with Aperture Science Equipment
Derpy is banished to the Crystal Caverns with Chell. Somepony wants to break her out.