I'm "herding" together some posts about various common causes of excess hair shedding because it had been on my mind (and my scalp) lately.
Firstly, I updated the "Friction and Shedding" post, which could also be titled, "Are your hair products ripping out your hair?" Too graphic?
Secondly, The post about iron, vitamin D and hair loss - these are common deficiencies and common causes of hair loss.
Thirdly, bringing back the post about seasonal shedding. Not everybody notices this. And if you have the other two causes of shedding (above) this one might be more noticeable.
This is not to leave out the other causes of hair loss that are significant (illness, medication, severe stress, scalp disease). But other sources have more information than I do on those things.