That's a great story. I might treasure that returned card for a lot longer than a normal one, just as a conversation piece. As a Christmas celebrating non-greeting card participator, I usually just think "oh, how nice of her/him/them/it", then throw the thing away. If it's from someone I haven't heard from in a while, I might call them and thank them.
That professor was a jerk, but he might be one of those people who's very literal-minded and doesn't understand that there are emotional connotations in returning something to the sender, even though it might logically make sense to just send it back if you don't want it. In other words, the kind of person who doesn't understand the idea of etiquette.
I don't know the man, so he may be a jerk, but it's possible he's not. I'm pretty literal minded myself, and I can assure you, I do understant the idea
of etiquette. It's the mechanics of etiquette that often escape me. The idea of etiquette is that other people have feelings and priorities and lives, and that I shouldn't live my life without taking those into account. It's just sometimes very hard for me to figure out what someone's emotional reaction to something will be, because it very often isn't at all what mine would be. That's why I haunt this forum.
Mr. Professor's thought process may have gone something like this:
Look! Something from co-worker, what could it be? Oh...a Christmas card, I've heard of these. Why in the world did she send me this? Am I supposed to send her one? Is she expecting it? I didn't know we we so close. Will she be hurt or offended if I don't? I don't want to hurt her feelings. I know! I'll send it back with a polite note, thanking her for thinking about us. Then she'll know I don't hate her, and I'll save her the time and expense of doing Christmas cards for us in the future. A perfect plan!
And then he sent it back, probably before his wife got wind of the plan and stopped him.