Being the Deity of Time is tough work. You've got to protect the timestream, save ponies that tamper with time and get stuck in infinite loops.
There's also a lot of experimenting one can get done with the ability to reset the universe each time.
a lot of experimenting.
Point is, she's done a lot, for a LONG time. So, after her latest experiment goes very wrong, she decides to take a vacation.
Back to where she was once known as Twilight Sparkle.
Also, Discord does some things too.