What snapped my fishing line?

Ok, I was originally fishing an area of a lake where a stream feeds into it. I saw some dark clouds and heard some thunder so headed to the dock. I was on a paddle boat so I could get into some small areas (So I wasn't to far from the dock). When I got to the dock my little cousin (2 years old) was sitting in a swing that was set up near the dock. He was sitting with my sister and other cousin. He kept yelling for me to catch a fish. So trying to please him, I switched to a rooster tail and tried to catch a bream or small bass so he could see the fish. On about my 6th cast cast something hit my line hard. I was only using about 5 lb. fishing line or maybe smaller (didn't expect to catch anything big). As soon as my line got tight it snapped. I want to know what hit it. It wasn't but maybe 10 ft. from the bank. It didn't jump or splash, just grabbed the rooster tail and took off. We have caught some very large gar out of that water (what I think hit it), but there are also bass, bream, carp, and catfish. Any idea what actually snapped my line that quick?