Here is a video of thread being removed from the upper tension of a Bernina sewing machine. As you see in this video notice that it doe not take much thread to create issues with your sewing. Also notice that a machine needs to be taken down to the frame in most cases to remove it. What is not shown in this video is that thread also wraps around the take up lever assemble too. This will cause your sewing machine motor stress as it sews. It also places a bind on the mechanics of the machine to. If it is not removed it could be fatal to the take up assembly and motor. As the video show these thread wrapped about the tension it does not allow the tension discs to close properly, therefore you will have inconsistant tensions. What is interesting is that these broken threads will float around the tension discs axle as you sew, so one minute your discs closes properly and it sews great, then the next minute if floats back to making your disc open up creating non-tension issue. The only remedy is to remove ALL the broken threads. So you ask how did they get there to begin with. As you sew your thread will break on occasions. These small thread fragments will hang up in the mechanics of the machine, then you proceed to sew again and they wrap themselves around the assemblies. I hope this has been helpful.