These are hairs that were growing from your head and are let go by your follicles. It is a completely natural process, the same as sloughing off dead skin cells.
There are 50 to 100 hairs a day that could be being "let go" by your hair follicles. You usually find them on your shower wall or on the seat of your car but most end up in your hair brush.
When you wear hair extensions, many of these hairs shed remain caught in your bonds and stay there until you remove them.
So, at 50-100 a day, the shed hairs may not be as visibly noticeable when removing your extensions after a few weeks, but become quite noticeable after many, many months, resulting in a "hairball." Be sure to look over the articles identifying naturally shed, and unnaturally shed, hairs so you can easily identify a "safe" hair extension system.