Drains get clogged for all kinds of reasons. Even if you are diligent in what you dispose of down your drains, over time they can still become clogged.
With regular use sediment can build up in the drain restricting the flow of water. This is common in bathrooms where hair and soap scum can stick to the interior of the drain pipe. Over time you'll notice that water takes longer to drain than normal. If you suspect your drain is clogged, get it inspected and/or cleaned sooner than later. The longer you wait, the more costly problems can be down the road.