My first year as an ALT I was almost transferred. I hadn't done anything wrong, but the parents grouped up and went after the English department for their incompetence. The students' mock exams at their cram schools were absolutely abysmal, and so naturally the parents flipped a lid on the English department. Since I was the new guy there they decided they'd make me the problem.

I had an MA in TESOL at the time, but the teachers' curriculum was essentially the audio-lingual method that linguists found didn't work back in the 1970s. My performance reviews were dynamite until the one they tried to point at to get me canned. Expendable ALT, easy target.

Anyway, that job got me a visa which was all I cared about. Fifteen years later I got my Japanese teaching license and now I've taught at colleges/private schools. Trash schools will do trashy things, although I agree it's unlikely that OP was given no reason. I just thought I'd chime in with an ALT being removed for less than valid reasons.