here is scoop from tandanu. it is addon combination of LibJostle and DBM. so basically, what you said Phanx. :) It would probably need to be fixed in LibJostle. It's the actual origin of the taint.
""moving frames down" means it would try to move the WorldStateFrame (the status display in battlegrounds and some instances like MH). The Battleground mod tries to anchor the "click here to target the flag carrier"-frame to this frame which is a protected frame as it performs a secure action (targetting something). Everything that controls the state (visibility and position; the WorldStateFrame controls both as it is the parent of the DBM frame) of a protected frame will also become protected.
Tainted code (addon code) can't perform certain actions (like showing/hiding or moving) on protected frames while you are in combat. The WorldStateFrame tries to perform some of these actions like showing itself which is no problem as long as it is untainted/secure. LibJostle taints the WorldStateFrame and it can no longer perform these actions.
Just having DBM installed is no problem: WorldStateFrame is protected but the code is still marked as secure
Having LibJostle installed without DBM is also no problem: WorldStateFrame is tainted but not protected
Both addons together will cause problems in some cases "
I just don't see how dbm can be causing the taint off hand unique to you when no one else seems to be getting it yet I do see the functions in the code that are referred to in taintlog so I don't really know. I'd honestly suspect either an addon conflict, or a damaged blizz UI addon from a recent wow patch though. That or you just found the perfect set of circumstances to produce a problem no one else can. Even when I do pre mades with guild, none of us have problems in any battlegrounds from DBM. Xperl on other hand. It still tries to set attributes constantly in combat and causes a lot of taint.