Most of the fishing line that ends up in the water is the result of a hook getting snagged on unintended objects such as rocks or tree limbs and the line breaking when pulled. Sometimes the line can rub against a sharp object and break. Additionally, large fish can sometimes break lines while they are being reeled in.
Even fishing line that has been thrown in the garbage can end up in the environment - either by blowing out of the garbage can, being taken out by birds for nest-building materials or removed by other animals. Some people also just throw the line right into the water when they are done with it.