I'm not a writer so I can't write a better description of a circle of suck than this by Ryan Nanni of Banner Society:
Any time you can build an ouroboros of football teams beating one another, that’s a Circle of Suck.
To put it into algorithmic terms (my favorite terms): suppose there exists a directed graph whose vertices are sports teams in a given league, and whose edges are contest results in the direction of the victor. A cycle in this graph is a circle of suck.