The reason is probably this, after so many write ups the bus driver is obliged to issue a 'kick off' slip. At our school it was 3 warnings then a 10 day suspension. Your school may be different. I also gave verbal warnings before issuing a bus slip.
For the time being you may have to wait it out. Then when he is allowed to ride again, speak with the driver and see if he will be allowed to sit up front. I only had a few spots that I could put kids that needed to sit by me.
If at all possible talk to the bus driver, create an open communication line. Give him/her your home phone number so if there is a 'verbal' type warning you can deal with it right away. To prevent future bus slips. If a bus driver feels that warnings are not working, that is why they give the kick off slips.
Honestly, the object of the bus slips is 'supposed' to force the parents into 'making' the child mind the driver. But in truth a child is going to do what they will, no matter how much punishment the parent enforces at home.
It seems as though everyone wants to pass the buck when it comes to raising a child. As a driver I tried giving as much leeway as possible to a child. Sad to say not everyone is as tolerant as I am.
As a possible solution to your dilemma, something I had to do with my oldest son, make him walk to school. Yes, I know this sounds harsh, I bundled up my other children put them in the car and followed with my flashers on, while my son walked to school. It helped him appreciate his bus ride and after that I never had another bus issue with him.
I wish you the best of luck! I hope this helps answer some of your questions.