Some troops that are above their National Morale gradually drops it to 'fine tune' to the National Morale.
I know that troops that get routed suffer NM loss. And troops that are isolated do so. It was not the case (Albeit I've experienced that and it's a hammering in morale, saw a Panzer Division that started a single turn isolated drop from 86 morale to 80 at once just by being isolated).
I am not sure if a 'low' supply status may induce morale loss. (By low I mean that at the start of the turn, post logistic phase the unit has 60% supplies - which is ... kind of normal due to Axis logistics once they're deep in Russia and not in Winter yet).
Then I saw also the Italians that freshly arrived in T8, started to trickle morale down to 40 from T9 onward whilst marching in Bessarabia. (Albeit they're also set as Priority 1 for Supplies). The Italian infantries start both with 50 NM when arrived on map. And they are losing 1 point every 1 or 2 turns roughly.
So I feel I am missing out on something - I've also German infantry divisions that were worked hard to Morale 80+ that are now back to the 76-77 league. Attached an example of a division with 0 defeats, 15 victories that barely has a notch above the NM and was higher than that before. - I can impute to it the supply situation (Closemost operational depot is Pskov) but this is just an example of many.

