Truthfully, you may have had this problem anyway. SOMETIMES a chick raised with an established rooster, never becomes aggressive, and sometimes, he wakes up one day, and thinks today is the day I am going for the top spot and the fight is on.
I try NOT to separate my birds, it causes a lot of problems. But with the days getting a bit longer, and sap is rising, the younger rooster may have been just biding his time.
If I was going to put him in a cage, I would put him with the other roosters. I am assuming that the other roosters are somewhere else? A bachelor pad of roosters, or the some other way of culling him.