I double checked all of the IAS equipment setup: re-leveled everything, made sure all equipment was over the proper monuments, and checked that all required lines were properly sighted (especially important for the Brunson). With this done I remeasured the pitch and yaw of the ETMy, the results are:
The yaw reading is consistent with the one taken yesterday. The alignment equipment is set up properly, so I think this change in yaw is real and not a result of improperly set up gear; this also means the source of this yaw movement is still unknown.