I tend to heal my army before I feed them to regain their morale. Twice now (once today, once yesterday) when I've healed them, their morale has magically gone back up to 100% along with their health without me having to feed them. The first time I thought I was just delusional. The second time I figured I might be delusional, but that was a bug.
I'm pretty sure both times were after running dungeons. Not sure if that matters or not, but that was the other similarity I could think of. That or my army gets some strange morale boost sometime between the hours of 11 am and 1 pm EST. In between I ran raids and had to heal and feed them as normal.
If it happens again or remember/notice anything else, I'll let you know.
--Az