those goofy teenagers

I have had 3 juvenile cardinals, and 3 juvenile robins hanging out in my yard. They are all eating on their own now, and get along, sharing the pond for bathing. Yesterday I saw a hummingbird at one of my lilies, and to my great surprise, one of the juvie cardinals drove off the hummingbird, and even followed it as best he could as far as I could see. This didn't make sense to me, since I never see cardinals on the offensive, and hummers and cardinals don't compete for the same food supplies. Are there any thoughts about why this would happen? In birdwatching, there is always something new or unexpected, it seems.

Susan in Minneapolis

Not sure? Just goofy teenage behavior?