What's causing the shadow running down the side of my photographs

I had a very similar thing happen 20ish years ago on my old Pentax K1000, the tensioning mechanism on the curtains was damaged and the rear curtain was traveling faster than the front curtain. Not an issue at slow speeds where the front curtain could fully open before the rear curtain started to close, but at high speeds the rear curtain would "catch up" with the front curtain before the front curtain had fully opened.

My situation was that over 1/250 the curtain would catch up about 55% across the length of the frame.

Sadly, I didn't know this and got back from a month-long overseas trip with about 1/3 of my images missing the right half.

Obviously not quite the same thing, but there are a few other mechanical issues that could cause similar situations with out of sync curtains. For example, the curtain "catching" at that speed due to the position of the internals and getting jogged into its final position by the rear curtain closing against it.