That Gazette article just says a union, it doesn't say which (although it may have before).
I would imagine there are a fair few ways to argue this is public interest:
The fact that they are an elected official (in the same way as an mp).
The influence JB has with regard to the GRA and training organisation.
While the things we know about are apparently legal (don't know full details) there are often more incidents. It is possible there were criminal acts and publicising these incidents may help victims come forward (this is one of the arguments against giving rape accused anonymity [ please note I am not in any way suggesting JBhas commuted rape, merely that it is precedent for publicity]).