There’s etiquette involved in mooring-up to a post or cleat that’s already occupied. It’s called “dipping the eye” and if you don’t think it important let me tell you it was the subject of a thirty-page discussion on a boating forum just recently. I use the term “discussion” somewhat loosely.
Anyway, the procedure is shown in my sketch. The most recent arrival passes his line through the loop of the incumbent’s line, then over the post. Thus, when the incumbent wants to depart, his line isn’t trapped by the new arrivals line. Simple, really.