Originally Posted by
FlatMaster
Oh!! I think I firured something out. Perhaps mechanical precession. The bolts on a chainring are right-hand threaded so mechanical precession acts in the tightening direction. Now that you're fixie, The mechanical precession of pedaling backwards would act to untighten the bolts.
The OP's problem is caused by incorrectly spaced out chainringi bolts or incorrect torque.