Could you maybe link a video in this thread? It could be quite easy to diagnose based on seeing your technique?
Usually this problem has 1 of 3 causes, or a combination of them in the worst cases.. 1) your back has a curve in it putting your feet behind you (farther from the rope) and is caused by bad diving posture which usually requires a lot of FIM to correct. 2) you are bicycle kicking/too much knee bend, and the lack of efficiency in the front kick forces you to keep your legs far from the line so that you don't tip over into the line. Or 3) (this requires video analysis) you have some other alignment issues in the amplitude of your finning (ie; faster back kick, more powerful front kick...)
One of the main causes for this is lots of shallow snorkeling or pool training without enough weights. Both of these activities require a legs above the body position, or one of the other kicking issues I mentioned to help fight buoyancy and stay below the surface. Obviously in a vertical position the buoyancy is parallel to your direction, not perpendicular, but the technical habits of fighting perpendicular buoyancy will remain.. and there for push you into the line.
Another common cause is fins that are too stiff, too long, or with too much of an angle between the blade & foot. These can also force big knee bend and cause one of the above mentioned problems..