The sensor heats up during long exposures, turning parts of the edges purple.
In other words, it's normal. The exact location, number, and severity of the purple spots depends on the camera.
On my star trails shot, which coincidentally was also taken in Vermont, I fixed it in editing. Mostly, anyway. If you look in the lower right corner, you'll see that there's a slight change in color - that's what's left after basic editing. You can also see a similar blotch along the top edge just to the left of center:
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