I see what you mean. In this case, I think Bill was concerned that the forum itself might disappear, as well as expecting he would stop participating regularly. And thus, wanted to establish other comm links and say farewell otherwise. I hope I have that right--that's my recollection. Also, I've been happy to see him back around somewhat, always enjoy his company, insight and experience.
More generally, and perhaps more to your point, I have observed that folks sometimes get frustrated when participation dwindles, and they sometimes announce a goodbye as part of that.
"I don't have time to get into it, but he went through a lot." -Pulp Fiction