Could it have also been a bit of jealousy on Tony's part? Junior was more of a father to Tony than his own father but he never really got the love he wanted from Junior. "He never had the makings of a varsity athlete." Maybe seeing this asshole, who was about his age, being his Uncle's main guy, someone Junior clearly respected more than Today, hurt his feelings in some way, or being a dick to Mikey was a (subconscious) way of getting at Uncle Junior.