I have heard this many times. Although it is fairly rare, if you spend enough time you will hear it. For 15 years I was a radio telegrapher at KPH, spending 40 hours + every week, and that's a lot of hours!
This happens on all bands. Like you, the first time it happened to me, I thought some lid was tuning up on my frequency jamming me... I can't tell you the ins & outs or exactly what is happening, just that I've heard it many times.