Watch: Rory McIlroy confronts a caddie

McIlroy angrily confronted caddie Jim "Bones" McKay in the parking lot after the second day of Ryder Cup play. Reportedly, McIlroy was angry at Patrick Cantlay's caddie, Joe LaCava, because he was waving his cap in celebration after Cantlay made his putt on the 18th hole.

LaCava waved the cap and then didn't move out of the way as McIlroy was lining up his own putt. When McIlroy's teammate Shane Lowry started yelling at LaCava, the caddie yelled right back.

The European team jawed with LaCava on the course, but McIlroy was still so heated that he got in the face of a totally different caddie after completing the course.

Cap-waving has become a phenomenon at the Ryder Cup because Patrick Cantlay isn't wearing the Team USA cap. It's been reported that Cantlay is protesting that Ryder Cup golfers don't get paid, but his agent insists it's merely because he couldn't find a cap that fit him comfortably.

Cantlay said it's simple. "It didn’t fit at Whistling Straits, and didn’t fit this week,” Cantlay told reporters. “Everyone knows that."

But fans got into his hat-less-ness, chanting, "Hats off," when Cantley teed off and removing their own headwear. When Cantlay and his partner Wyndham Clark defeated McIlroy and Matt Fitzpatrick, the members of Team USA waved their own caps.

So it's possible that McIlroy is upset about a terrible breach of caddie etiquette before a meaningless putt. But it's also possible he's more upset that he capped off a big day for Europe at the Ryder Cup with a close loss.