Why do I get worried about my friend when her parents fight?

My friend and I are both 15, turning 16, anyway her stepdad use to come home drunk all the time and start fights with her mom and things just got so bad to the point that she, her mom, and her little brother and sister had to sneak out of the house one night because he was drunk and was pretty much holding them in their house and blocked all the doors so they couldn't leave. Then when he realized they were gone, he went out looking for them.
Anyway, in this case though, her stepdads sober, but she sent me a text a little more than an hour ago that said "I gotta go.. there's a lot of fighting going on right now.. I'll talk to you later" and even that got me worried. I just don't know why, when there's fighting I just get worried about her, even though her and I go through similar things.
Deep down, I know she's going to be okay, but I still get knots in my stomach and get really worried anytime she has to go because her parents are fighting (they don't fight as much as they use to though, thankfully).

Help, why do I get so worried? :/ and how can I stop being so worried?

There's nothing wrong with getting worried. It's just something natural that happens when there's something dangerous happening. The only thing I can think of to help you stop worrying so much is to just try and focus on other things. Watch something you're interested in, play games, listen to music.. etc.

Behind worry is usually fear. Fear has a physiological effect on our body by releasing certain chemicals into our bloodstream. These chemicals help us to run or fight - both involve movement which then releases the excess chemicals and our systems start to calm down. In your case because of your close friendship, anything that threatens her world threatens your world. The problem is she has the need to run or stand up for herself because she is physically present but you are powerless in the situation because you are there with her in spirit but physically it is not your situation.

My advice would be next time you feel panic or worry on her behalf do something intensely physical. Find a flight of stairs to run rapidly up and down, ride a bike and pedal as hard as fast as you can. It needs to get your heart rate up and be intense enough that your mind is focused on the activity. If it helps, picture yourself escaping from an angry man and getting help or going somewhere safe. Exercise will help to release the build-up of chemicals in your blood-stream.