Mysterious Morale Loss

[Warmonger]

What causes units to lose morale from one battle to the next during a campaign? I am playing the long WWII campaign as the Germans, and just completed the first battle versus the Polish. I lost two units. All the stats for these two units changed...some up, some down. (I assume these are fresh "rolls" for the new leader, using the base experience and morale, etc.) I also upgraded some Rifles into Engineers, (cuz I forgot to buy them!), and they (appropriately) lost experience while other stats showed the usual slight gain. Now comes the mystery:

Several units lost morale, for no apparent reason. They survived the battle. They didn't take heavy casualties. (A few did, but others were never even fired upon!) They are of various types and from various formations ... I just can't see any connection.

Tanks in advance for your insight!

P.S. As a matter of fact, insight on ANY campaign stat changes is much appreciated!