Playing ver. 1.21f.
I first noticed this behavior several games ago after the initial landgrab had run its course and I was ousting a minor civ from "my" continent.
After razing ill-placed AI cities, I was waiting for my new settlers to arrive on the scene, but transports from several different civs beat me to the area, one at a time, over the course of several game turns. Since I had quite a few cavalry in the region, it was easy to knock off the newcomers. The AI just couldn't seem to learn that this was going to fail, and they kept trying.
I don't remember whether there were resources involved. I wouldn't be surprised, as they do seem to key on them.