I think it is incorrect to attribute any specific cause. My understanding (admittedly, limited) is that some, like Bermuda and HpstrDufuz, didn’t make the transition over to the new boards from AOL. I am told that they post in some of the other Straight-Dope related websites.
RngrJeff had to change his email name, he is now Yer Ranger, and his job and personal life have been taking their toll on his online time.
I suspect there are many different personal reasons. My assumption is that people who leave because they are angry with the moderators or frustrated by the software, say so. There are very few such farewells actually posted; so I think the majority of departures are from other individual personal reasons.