I don't think it is as much mental as some are saying... When tapering bupe, I would be able to feel exactly how long it was since my last dose just by how strong my symptoms were. Sometimes I would forget to dose and sure enough, a few hours later I would get an ugly reminder.
I agree that if you didn't realize that the bad feeling was caused by withdrawal, you could attribute it to other things, but you're still feeling the pain regardless.
Activity while in w/d provides some relief, sure, but I'd attribute this to physical changes caused by activity. Similar relief comes from things like hot showers.