Read here about "reaction scanning" describing the phases of the horizontal scan process on a CT-100 (and likely most other tvs with a flyback transformer).
http://www.earlytelevision.org/Deksn...implified.html
I suspect that the "drive line" is visible when the transition between the various scan states is not correct due to leaky caps out of spec resistors, weak damper or HO tube, miss adjusted grid drive, while BO is a self oscilation of some HO tubes, which can indeed be stopped by a magnetic field from an ion trap mounted on the tube. Two different causes of lines, IMHO.
jr