I’ve never been let go from a job but if that happened I definitely would need to know why. And in most cases people do know why. Unless it’s a lay off due to cutting budget and the only reason is that they had to do massive lay off. In my experience with people who were let go, they always knew why.

Now in your situation as you weren’t employee of the district and were subbing, you weren’t really fired by them as you didn’t directly work for them. I’d see how they sometimes wouldn’t tell you.

You are just being put on “don’t come back” list. Yes it is always sudden and without warning. You don’t work for the district so they wouldn’t give warnings. If that happens you likely wouldn’t know the reason in order to protect staff who asked to exclude you from sub list for that building. Likely teachers you subbed for or office employees or even parents. Then administration makes a decision based on that.

You can try to remember what transpired at that time and what events led to this outcome