There are a lot of reasons why hair falls out. One of them is that we, like other animals have shedding seasons. Sometimes, your hair starts falling out more as the folicles end their cycle and release hairs.
A change in hair care can also cause hair to fall out.
When I started washing my hair every 3-4 days instead of every other day, I felt like my hair was falling out a lot as it came out in the shower but that is because when you wash your hair, all the hairs that have been shed but get stuck/tangled in the rest of the hair are released. I was only noticing it more because after 4 days, more hair was trapped in the tangles. Certain shampoos make my hair feel limp, thin and very frizzy. Doing conditioner only washing has improved that greatly.
like amy said, there are medical reasons as well such as stress or alopecia. you can try taking vitamins or go to see a doctor if you feel it is falling out at alarming rates.
The more you worry about it, the worse it can get as stress wrecks havock with your hormones and can cause you to have bad hair and skin.
If its not falling out from the root then it is damage. you can't really fix that apart from with a good cut