What you pictured is definitely a tension issue. Not only are you getting the bobbin thread pulling up at the pivot point of each swirl, I am seeing eyelashing on curves, pokes of bobbin thread coming up on areas as well as a bit of top thread "flat lining" where it isn't forming a good balanced stitch. These are all indications your top tension is too tight or your bobbin is too loose. One trick you can try on your bernina, check your bobbin case, some models of Bernina have a little hole at the end of the "arm" on the case. Once you insert your loaded bobbin and thread the case, run the end of the thread through that little hole. Then insert the case and bring your bobbin thread up. This puts just a teensy bit more tension on the bobbin thread. It is mostly used for doing machine embroidery or satin stitching to prevent the bobbin thread from poking up to the top when doing this kind of dense stitching. Sometimes just running it through the hole is enough. But based on your picture you will probably need to tweak your tension settings as well. I would start by just reducing the top a smidge.