Tree's roommate, Lori -- initially the most innocuous of those in her inner circle -- is to blame. She's jealous of Tree's romance with Gregory. So she funnels her hatred into the poisonous cupcake Tree's gifted time and time again. Why did she die after killing the inmate? Tree finally pieces the puzzle together after the fake-out happy ending: The last thing she did before falling asleep was share the cupcake with Carter.
Solving the mystery and thwarting Lori stops the time loop, but it still doesn't fully explain why it happened. For that burning question, you'll have to hang your hopes on the future, like Lobdell and Landon.
"If I'm so lucky to get to make a sequel, and we would never count our chickens and I have no idea what's going to happen with this movie, I have the sequel ready," Landon says with a laugh. "It gets into the [time loop's] why, in a very, very, very, very specific way."