He "retired" during training camp before the 1980 season and sat out that year. I really don't remember why he did, other than Riggins was the kind of guy to do whatever the hell he wanted to. Sound like Ricky Williams?
Gibbs went to his farm in Kansas the following spring to meet him and see if he wanted to come back and play. He never met Riggins, but did meet his wife, who Gibbs told that Riggo was welcome to come back if he wanted to.
Riggins showed up in camp about a month after that with the immortal line: "I'm bored, I'm broke, and I'm back."