My take on this. It's quite likely there was a clause in his contract back in 2016 that said he had to inform them of any police investigation/arrest/question under caution. The BBC knew nothing about it until the alleged victim got in touch very recently. So that's probably sufficient grounds to terminate his contract. I guess there will also be clauses about personal behaviour/bringing BBC into disrepute, but I'm not sure (from what's currently in the public domain) that there's enough to sack him immediately if he were employed. But if his contract is for services through his production company, then employment law/employee rights don't come into it. So it may be that just the allegation in the press is sufficient.
It also potentially puts new context on his previous partners suicide - if he became aware that SM was cheating on him with an underage boy, could that have been part of the reason for his suicide?
Also, how many times has a celeb been outed like this with a single accuser, and then more victims come start coming forward? Not saying this will happen with SM, but there's a distinct possibility.