One thing is having a student who is problematic and violent in your classroom, but a completely different thing is how you react to  handle the situation, and if you got punished by your bosses and the authorities of the universities who hired you in the first place, there are high chances you did not handle the situation in a proper way and you actually made things worst after the student started to act out in your class.
If a student threatened violence against me in a classroom , I as a teacher I would never directly engage with the student, but rather call for the university security to get into the classroom and physically remove the student out the class, so I could continue teaching.

Perhaps, you did exactly the opposite and directly engage with the problematic student, resulting in you getting punished for acting out as well on from of a whole class.

Just my guessing, of course.