Tommy Robinson was convicted for contempt of court because there were a large number of linked trials taking place and so not to bias juries in each of the trials, reporting restrictions were put in place so they could get a fair trial and if found guilty which they were , a solid un-appealable conviction.
Tommy Robinson set up outside the court filming live on Facebook the people attending the court hearing thus jeopardising the whole number of trials . This is why he is in jail.

Thankfully. The defendants were all convicted and got long sentences but it could have turned out so different due to Tommy Robinson and his disregard for the welfare of the victims. He should have got 10 years.