OK, here goes, and it's weird. BLUF-- send a ticket.
He was signed to a long-term contract with a mutual option a couple of seasons ago. The player declined his side of the option at the beginning of this season, which (I think) should have made him a free agent. However, since he had less than 3 years of major league experience at the time, the game engine decided he wasn't eligible for free agency and signed him to his team at the veteran minimum of $500K. Once he became unassigned after being traded, presumably the game engine decided that he had no contract, because all he had was a declined option.
This is a bug. Send a ticket.