Pure speculation on my part, but I'm guessing he got a significant ownership stake in either the Dallas franchise, or the league, or both.
He figures, "it might be Amazon at $3, or it might be Sears at $35... I'll make multi-generational money (not likely), or I'll have some no-pressure fun. WTH...I'm in."
I'm sure it's a lot of money for a zero pressure job where he gets to be around the game. Win-win all around... Same reason Spurrier and other are doing the AAF.