When your brushing/limbing, if your chain isn't very snug and you don't get the rpms of the saw up before you hit that twig/branch/brush it will grab the chain just right to flip it off the bar...trust me, I learned this brushing many a time at an old job
Changed the Chain. Now cold has set in, and had to go back to work! Another project that will have to wait a while. And my old truck wont start!