If you look back at Sasha's relationship with John Nicks (Sasha called all the shots, and always did things her way) and compare that to Sasha's relationship with TT (TT strict and demanding, Sasha obedient) it was pretty clear that a clash with TT had to develop at some point. What caused it to happen may never become known.
Look also at Sarah's relationship with Robin: she often said they were as close as sisters and she referred to Robin as her 'best friend'. The relationship had developed over years, since Sarah was a young girl, long before she became an elite skater. They shared a goal, and they worked toward the goal cooperatively and with equal enthusiasm. Last season, though, Sarah seemed to be a reluctant competitor, and Robin couldn't motivate her to work and practice as vigorously as she had pre-SLC, and the results showed at Worlds 2003.
As a coach, Robin allowed Sarah to continue with poor pick jump technique (that high kick) and the flutz, and didn't start working on the posture faults until fairly late in the game. Sarah's athletic prowess and consistency plus her determination work hard and succeed was probably just as much of a factor in her success as Robin's coaching.
Now we have Sasha with Robin. This is a relationship formed overnight, and IMHO, these two are mismatched. Sasha sorely needs a technical coach, and Robin is not that. Sasha also needs a coach who can handle her headstrong tendencies. I am not sure Robin is all that forceful. She couldn't get Sarah to buckle down and practice in her final season (and it seems Sarah has hung up her skates for good, if Robin has accepted Sasha as a student).
I think Sasha was looking for a coach on the East coast who could help her win the titles she hungers for. Unfortunately, a coach cannot go out there and skate for Sasha in the big pressure events.