Older parts of Hull have Victorian clay drains that are over 100 years old. They crack with ground movement, tree roots get in, and eventually they collapse or block solid.
Tree roots seek out water, and if there's a crack or poorly jointed clay pipe, roots will get in and spread. Over time they form a solid mass that blocks the drain. Common in streets with mature trees like Newland Avenue and Princes Avenue. Jetting clears them temporarily, but they'll come back unless you replace the pipe.
People flush wet wipes, sanitary products, nappies, and other solid waste down the toilet. It gets into the drain and catches on rough joints, tree roots, or dips in the pipe. Eventually it builds into a solid blockage. Don't flush anything except toilet paper and human waste.
Pouring grease, fat, or cooking oil down the kitchen sink is a major cause of outside drain blockages. The grease cools and sets in the drain, catching everything else. Over time it builds up like arterial plaque and blocks the drain solid. Scrape grease into the bin, don't pour it down the sink.
Ground movement, subsidence, heavy traffic, tree roots, all can cause old clay pipes to crack, collapse, or shift out of alignment. When a pipe collapses, waste can't flow past and the drain blocks. The only fix is excavating and replacing the damaged section.