I think it was just old age, he was about 2 years old. He had been steadily declining over the past week or so and had been in a coma state over the Thanksgiving weekend.
He was such such a sweetie and love to give ya smooches
I got Kyle , Butters (who passed away at the end of Sept) and Stan who is the sole survivor of the 3 together around Easter of 2007. They were all the same age. Stan still seems pretty healthy I may add.
Here is a pic of Kyle when he was younger