They agreed on $4k and Kuchar ended up giving him $5k.
Those caddies make about $100 day normally. I have no problem with it if that side of the story is what happened. You agree upon an amount beforehand and live up to that agreement.
Most likely, someone got in the local caddie's ear and told him what generally happens and he wanted to get his hands on more money. Kuchar's agent offered the guy $15k more I think which Kuchar didn't appreciate because an agreement was already in place.
It's tough, but Kuchar is widely known as the second most generous "tipper" on tour behind Phil. Knowing what I know about Kuchar and his reputation, I think the caddie is the real issue.