It could be bad threads in the riser.
More likely it could also be a lack of high pressure grease down near the bottom of the threads. When you thread a bolt that deep the grease tends to be removed from the bolt by the time it is screwed all the way down. Even on standard length limb bolts this can happen. I have had to fix a pile of bows with this problem. The fix was a Q-Tip or brush to pack grease in the risers threads all the way down to the bottom of the hole.
BTW, I have never had this problem with any of my Bowtechs either.
