That's funny! That happened to me once, not with yarn, but with books, on an almost empty flight. They slid all the way to the back of the plane when it took off. I was so upset because it was a pretty short flight and we wore our seat belts the whole time so I couldn't get up to get them!
Something like that happened to me on a New York City bus. Except that EVERYBODY noticed when the ball of yarn rolled all the way to the front of the bus. It turned into a real circus, with people trying to be helpful and pulling the yarn back through the seat bottoms and getting tangled up with each other and the yarn. And fortunately, the floor of the bus wasn't TOO dirty.