There are many things that can cause clogs in your drains, but few clogs are more difficult to deal with than those consisting of grease, since they are naturally water repellent, making it harder to break them up.
The problem is that when you flush grease down your drain you might think it is all right since it is in a liquid form and seems to run ok. What happens is that as soon the grease hits the cold water in your pipes, it starts to coagulate. If you are very unfortunate, this might not happen until it has travelled some distance down your pipe, making it that much harder to get to.
Instead of pouring grease down your drain, you should pour it into a container where you can let it cool down and then scoop it into your garbage. Even small amounts of grease can build up over time, so you should get as much of it as possible out of your frying pan before washing it in the sink.