It most likely comes from a forseful twist of the arm!! To add to that: it's caused by an extreme muscle contraction, in other words it's actually your arm muscles that break you upper arm bone! the...
In a femur it would be something like a hard landing on your foot causing pressure and thereby creating a spiral as it cracked in an upward motion. In an arm is similar. It takes a lot of force to...
If I read your question correctly my answer would be heat convection. The air around the Bunsen burner is being heated thus it will rise and cause a vertical "wind".