Bigborgel
I think it's just his frustrations involving a lot of things.
Once these guys get a taste of the good life on tour, being relegated to go back to the next level down has to be quite the humbling experience, especially for someone trying to continue the pro golf lineage of his father who was quite successful on the biggest stage. Dad won the '82 Masters and went on to win 13 times total on the PGA Tour.
Kevin was ranked 52nd in the world back in 2014, shortly after having won his only tour event that season - the Phoenix Open. The win got him into the Masters a few months later where he carded a very respectable T8 finish. Then came the broken bone in his hand, followed by the anchoring ban which was implemented back at the start of the 2016 season. He's not come remotely close to earning full-time PGA Tour playing privileges since... partially due to his hand injury, the other part due to his putting.
He's won on the biggest stage and played well in a major. To say his career is a bust would be partially untrue. But I think he sees the writing on the wall and knows that he just doesn't have the game anymore, or at least currently, to compete. I suspect he'll continue working to get to the next level for a couple more years, but I just don't see it materializing.