Why is there a conflict between Cambodia and Vietnam in 1970s?

I understand there were border conflicts, but N. Vietnam supported Pol Pot's Khmer Rouge in the late 1960s, and by 1970, Vietnam was unified with N Vietnam taking control, and Khmer Rouge overthrowing the Cambodian govt to take control as well. Both of them are communists, and worked together earlier. Why did they clash so much after that, and ultimately leading to the Vietnam Occupation of Cambodia?