Well, low profile tires tend to do that. When you're driving down a bad road, the car tends to pull easily over road imperfections.

As to why you're losing grip as the tire wears, I guess that depends on the manufacturer. Tire construction varies, and I guess Kumho decided to go the frugal route if you feel that your tires are losing grip as it ages. You shouldn't really feel any loss of grip unless there is no tread remaining on your tires.