Watson may not be the only backs player off (the extension could well have been for this year only, and he has been somewhat injury prone). I do wonder about Sirker, and some others.
I suspect Levi has gone because he had already been told he wasn't getting an extension. Facing no play next season, he was offered a medical joker contract, which for insurance reasons probably cannot be a full contract right now, but as soon as the season is over, they will announce him as a full signing. With such a deal on offer, he probably approached Wasps many weeks ago to get a release, and got it. Bad timing for Wasps, but there it is.