I'm leaning much more towards someone breaching privacy. Password protected files, non-public YouTube videos - clearly he didn't want his identity public for whatever reason. If those files and videos were shared elsewhere that might have prompted him/her to split. I'm not necessarily implying that they were an ex-developer or somehow had access to the engine. All I know for sure is that when people with a lot of knowledge about a game come in and try to help it's best not to ask questions. I literally have been modding games for 10+ years and have seen this happen a half a dozen times. Luckily I've kept tabs with a few ex-devs due to my journalism connections. Their response for abruptly leaving a forum? Someone got nosy and tried to figure out who they are.
Again, I dunno if this was an ex-dev or some other situation, but the next time this happens just appreciate the help they're given and respect things they ask of the community. I'd almost guarantee files or videos went somewhere they weren't supposed to. That's how it's gone down most of the time before.