Two points, either of which will cause a deviation of the compass
1) Think about the compass itself.
If it is exposed to a vast magnetic field that a lightning strike will create, it may itself become remagnetised in a different orientation (as per Nick's comments).
2) The magnet in a simple cockpit speaker will effect compass in normal situations, hence the need to have a compass swung and deviation cards created whenever substantial new equipment is added. Therefore, if the gearbox suddenly becomes a magnet, even a weak one, this will have a marked effect.
A secondary effect of point 2 is that you need to make sure that gear components have not become magnetised or else they will render the chip detectors ineffective as the chips may cling to the gear itself and so not get picked up.