We just moved out of a three level Townhouse in VA. We used the basin in the basement maybe 5 times in the two years we were there. Never noticed a problem. Now the landlord claims we put leaves down the basin and clogged it. We have no idea how leaves got in the sink. Our children are too small and we never brought anything other than laundry down to the basement. The neighborhood is in a very wooded area, bushes right outside the basement window. Is there a possible explanation for the leaves in the basin drain?

Leaves fell down the vent and partially blocked the drain. It backed up the main line into the sink and then cleared. Since you never used the sink, you never cleared the trap or line.