I don't know, but I think it is not likely that Sasha had any problems whatsoever with Anna. They are mortal enemies on the ice, yes, but somehow I feel that all the videos we've got with them together seem to suggest they are having great fun on the ice when they are together.
And that also really showed in all the backstage footage we got from Nationals and Worlds this year. The only two competitions they competed against each other. It looks like real friendship.
I think that Sasha left because she wanted to do more quads. However, there might have been some kind of situation where Alyona took up too much space. After all, Alyona was the most successful of the 3A in their debut senior season, and maybe Sasha (or her father!) picked up on that and decided to leave.