It happened recently. I saw it and I saw why all in the space of 4 and 1/3 minutes. A way back when I took the guide class for my BERNINA 1080 I remember the instructor telling us to clip the thread at the top and pull down in the direction of the thread path. Grabbing the spool and pulling back could cause a couple of problems and/or damage the tension discs. Pulling back can pull lint (cotton thread is linty no matter what, some more than others) and thread snippets back into the machine. Whenever I’m teaching, be it a machine quilting class or a guide class, I make sure the students know this bit of essential information. Then when I saw the tension discs all out of whack what happened was almost obvious. I mean something else could have happened, but that pulling back of the thread just about had to be it. And it was. Often this comes with two statements, “I’ve always done this.” and “No one ever told me not to do this.”
Usually what happens is thread gets caught in the thread uptake lever that is often pulled out with tweezers. Sometimes it takes a little bit more than tweezers to get the thread out. On some machines the side cover opens or comes off making accessing the thread uptake lever much easier. Hint, if you can pull the thread from back to front, it comes out a little bit easier.
So, please be kind to your machine. Pull the thread down and remove pins before stitching over them.