Here's a 2020 update on the mysterious interference, emailed in by a guy who read about this issue on my website:
"
Hi, I can give you a definitive answer on the rising and falling signal you were seeing on some of the cable channels. It is indeed Macrovision (later Rovi) analog copy protection very much like that used on most prerecorded analog video. I formerly worked for a company that made cable DVRs and I often used a Tek VM700T or a TDS3000 scope with the video trigger option to test this particular feature, despite my personal dislike for forms of copy protection.
The signal that was intended to upset the AGC in analog video recording devices was generated within the set-top box and was controlled by some digital status bits transmitted within the video stream of the source material from the network provider and passed through our headend unmodified. The reason it is present on some channels and not others is simply that some TV networks choose to protect their content and others don't. This was around 2010-2013; I'm not sure what the current state of things might be. Hope this clears things up if you had not already figured it out."
Executive summary: the interference looked like copy protection because it
was copy protection.
Regards,
Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html