Three rules of plumbing

What is the meaning of "**** rolls downhill"?
and when people say like this?

It means when something goes wrong the blame goes down to subordinates

The well off in medieval cities also tended to have their houses on high ground for the very same reason.

I met a fellow who worked while in school for his father's plumbing company. He repeated the 3 Rules that he learned on his first day:
1. Payday is Friday.
2. **** flows downstream.
3. Don't suck your thumb.