I think this is basically two fold.
1) He has continually moved up levels throughout his career and wants to try his hand at the highest level
2) His frustration with roster management/recruiting at the college level. He was said to be pretty despondent when DJ Wilson left early. This year he has two guys racing out the door to be 2nd round picks in Iggy and Poole. I’ve felt for a while that it may cause him to retire earlier than anticipated. Can’t say I saw the NBA move coming though
Recruiting seems like it would be physically and emotionally taxing. You have to put in hours, and hours, and hours just to have a recruit in your top 5. And then you're traveling all over the country during the season - and you're still recruiting at the same time, going to several games and practices of your top targets. Even though the NBA season is longer, I think it might be easier on the body and mind because you're not constantly trying to win players over - or trying to keep players from leaving school early or transferring. Sure, you can always send your assistants out on these trips. But you're not endearing any recruits without a visit from the head coach.