Because someone actually wanted me to put this up.

Okay, time for the wild theories. Discuss.

My first thought was that Tavi recognized that it was a letter from Discord, and wondered why Discord was writing to Fluttershy.

All I can think of right now is that either the dark magic from Razz that was inside Twilight has taken shelter within Tavi, or Tavi is falling to her negative emotions similarly to Luna. I think there's lines that hint at something like that. I'm probably way off, though...

Let's see... She's a teenager doing teenagey things. That should cover all bases.

--Spade

Jury is still out on this, folks!