Why would he "not" look like a professor with a doctor of sociology when he danced?

From the text:

He had always been a dancer, my old professor. The music didn’t matter. Rock and roll, big band, the blues. He loved them all. He would close his eyes and with a blissful smile begin to move to his own sense of rhythm. It wasn’t always pretty. But then, he didn’t worry about a partner. Morrie danced by himself. He used to go to this church in Harvard Square every Wednesday night for something called “Dance Free.” They had flashing lights and booming speakers and Morrie would wander in among the mostly student crowd, wearing a white T-shirt and black sweatpants and a towel around his neck, and whatever music was playing, that’s the music to which he danced. He’d do the lindy to Jimi Hendrix. He twisted and twirled, he waved his arms like a conductor on amphetamines, until sweat was dripping down the middle of his back. No one there knew he was a prominent doctor of sociology, with years of experience as a college professor and several well-respected books. They just thought he was some old nut.