It is often a slow process as hair, soap scum, food waste, and cooking grease conspire to narrow a drain until it can no longer move water with any normalcy. You will see kitchen lines clog from grease or what has been put down the garbage disposal, and bathroom pipes have a way of collecting hair and the like from personal care products.
Then there are the deeper issues. If a problem keeps coming back, it could be a sign of trouble in the sewer. In older lines, tree roots will find their way through loose joints or small cracks, drawn by the moisture in the soil. Once they are in, they trap waste and create obstructions that only worsen. Ellicott City has its share of historic properties and mid-century homes alongside new construction; some of these older dwellings have drain materials that have corroded or become rough inside. Add in some renovations with pipework from different eras, and a recurring clog can be far more involved than a simple fix near the sink.