I remember way back when wewe gave me another prompt where Bloom joined the Trix so I kind of combined that with your new request of Tecna taking over as leader.
The group was in a state of unrest to say the least. Their dearest friend turned to the dark. And for what reason none of them could really actually say for certain.
All they knew was that she was gone. Lost to the dark.
Popular theory around Alfea is that Darcy had used some sort of spell au that Stormy had threatened her into it. Or--most popularly--Icy had managed to convince the girl that she was worth zaidi on the dark side.