It does not sound like anything serious (from what I read, anyway). If they do not fear each other then it is no doubt a game. Amber (female) likes to stand outside the bathroom door, when I am in there, and swat at all the cats as they go in and out to greet me. And the cats often hightail it through the house after each other but then they turn around, switch the order of chace and come running back. (and some, especially Little Kim, are vocal about it). But they must like it because they always come back for more.