Since this question was first asked and answered, the reporting ban has been lifted, so there is information now available that wasn't before. The following is a summary of a BBC News article, and all the quotes are from that article.
Tommy Robinson was arrested, and has now been jailed, for contempt of court; specifically, for "potentially prejudicing a court case".
Specifically, he "was seen filming himself and people involved in the trial". He was then arrested for contempt on the grounds that:
His attempts to film defendants on that occasion could, the judge said, have "prejudiced" the jury, leading to an unfair verdict
Why did his contempt case initially have a reporting ban?
In this case, the judge ordered a temporary media black-out because he feared reporting Robinson's conviction could influence the jury in the very case Robinson was targeting.
The article provides some background on this subject:
Contempt of court is the crime of ignoring the court and its constitutional role in making sure that justice is done. [...]
Why couldn't [journalists] initially report Robinson's arrest and jailing? Reporting restrictions are a long-standing part of the British legal system. [...]
This is not some new form of censorship directed at Robinson. These are rules that apply to us all, equally. If he is unsure about that, he's now got time on his hands to read a copy of Essential Law for Journalists.