Thanks for your response. Deleting the drivers removed the problem but I needed the drivers to rotate the shoulder/collar bone when the arm rotates over the head, and so that the arm is above the shoulder when rotated over the head, but to the side when rotated down. The latter I discuss in Arm rotation 180 degrees: Is the arm out to the side or over the shoulder? .
I suspect this issue is related to dependency cycles. The shoulder has a driver which depends on the upper arm and the upper arm is a child of the shoulder. However, the upper arm only inherits the offset for loc/rot/size, and the shoulder being the parent does not affect the upper arm bone’s own rotation value. The shoulder bone only depends on the upper arm’s own rotation value (and not what the upper bone inherits from the shoulder) so I don’t think the cycle is really dependent.
I guess I’m going to have to redesign my rig so that there would be separate bones to rotate for FK, then put a copy rotation constraint on the arm bones to copy the control bones. If I make the top spine bone the parent of the upper arm control bone, then the shoulder would still depend on it but it would not depend on the shoulder.
Something I’m not sure about is if I should leave the IK chain how it is, or if I should make a separate chain for the IK bones.