It's actually pretty easy. First of all, some people might be very mean to you and mass-report you (That's how I got banned) and then Scratch Team sees this. They don't take time to look at your history, they only see the mass-reporting problems. So then they ban you. Second of all, Scratch Team is super sensitive and kind of overprotective about their already decaying community. So whenever they see something that might harm people in a mental way or something that is "disturbing" to others, they'd ban you.

Very. Just be nice, scratch is for all ages so a slip-up could give you a warning

You can get banned by making opinion that ST doesn't like.

It is pretty easy. All you have to do is break the guidelines 3 or more times or get reported tons of times.