The ribs of the flex hose are great places to grow tunicates and sponges. And they are great breeding gounds for chitons too. Its possible to partially clog that hose after a while. Then all you need is for something to get caught going down and start building up a slime ball. Just like a house that has a sewer pipe backup, it all starts with a tiny little thing that really amounts to nothing that gets caught in the pipe.