Flurry Heart is sick and tired of being ignored just because all of the other alicorns are grown-ups. What is she going to do about it? Free the Legion of Doom and take over Equestria, of course! And if that fails, she can always say that she only did it to try and reform those three pesky villains, and she might get a little bit of respect for that too.

Lord Tirek is not pleased to be working with ponies and changelings again, and plans to betray them.
Queen Chrysalis is only joining in because the end goal is in sight, and she can get her hive back soon.
Cozy Glow is planning revenge, but she just can't get over that awesome cute little alicorn. Wait, cute?!
Look out, Equestria.