Because he coached Corey Davis at Western Michigan for his career. And he was invited by him to be there. For Corey Davis he was his coach. The sign is paying the man his props. What's not to get?
Plus, they had at least 3 guys drafted and 2 signed as free agents I believe.
Although I wasn't really serious, you would think they'd just leave the Western Michigan part out of that big sign and just gone with P.J. Fleck. Corey Davis invited Fleck, yes. It's not like Fleck is representing Western Michigan, he was representing Corey Davis. Paying props doesn't have to include the Western Michigan part on the sign.