Rainbow Dash has been writing a novel about a mare who is totally not a substitute for herself. It's not bad, obviously, but it's not fit for publication yet. And some of the characters... miiight be a teensy tiny bit offensive to her friends.
So when she realises she lost it somewhere in Ponyville, obviously she rushes to get it back before anyone can read her messy (but still awesome!) first draft.
Now if only she had the common sense to keep quiet about her search, too...