Compression of the wood cells is the primary reason for the anchoring wood screw loosening. This happens everywhere - the anchor screws on the machine heads, the nuts on the machine heads, all the controls and face-mounted output jacks, plus the wood screws that hold strap locks and strap pins. When those wood cell walls crush from the pressure over time, it acts the same at the screw loosening. Once it gets started, it shows up on the strap anchors faster, since there is torque and other forces involved.
I think the reason it seems more prevalent on the strap locks is that people tend to feel secure since they are "locks" and don't check the pressure of the wood screw until its really loose. With the strap pins, you interact with the pin and your strap more directly than with the lock.