jsg What could cause this?
I suspect that your mount's RA axis had moved while you were tightening the altitude and azimuth locking bolts.
To test, do a Polar Alignment, lock the bolts, and immediately start another Polar Aligment, and look for the initial error of that second polar alignment. If the second initial error don't agree with the result of the first polar alignment, your mount's locking mechanism is the culprit. Try this a couple of times.
It is also possible that the locking bolts were not tight enough, and the polar axis had moved over the night as the payload moved around.
If the polar axis has moved while you tighten the bolts, you might be able to work around the problem by incrementally tightening the bolts as you are getting closer and closer to polar alignment, so that by the time you finished aligment, the bolts are also completely tightened.
If the locking mechanism is not a problem, check if your tripod legs did not sink into the ground over time. If it is, you can try to place the tripod legs over concrete stepping stones (that you can get from places like the garden department of Home Depot), instead of placing the legs right on the ground. (I actually also have a 10mm thick aluminum plate on top of each stepping stone, so that my tri-pier legs sit on smooth surfaces.)
Chen