There's thread creep, thread drift and thread lurch. I think that last was extreme thread lurch, where a poster desperately heaves at the tiller to move the thread to aiscussion of something important to them, but totally at odds with the thread subject.
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This here is thread drift, where the subject reaches out to related areas -
The Disbanding of Shit Shovellers Local 232

I'm reminded of the introduction to the old army Colonel Bogey song -

"We are the shit shovellers shovelling shit on the shit parade.."