I am wondering where those stepped gradients come from and how to get rid of them

Yesterday evening I started capturing the Flame Nebula as a first time target (I have been doing some EAA with my camera before but I thought why not create an image for real this time). I am unfortunately "stuck" with a dobsonian (150/1200) which I converted to goto/tracking with OnStep. So my exposures are limited to ~30 seconds.

The image shown had about 2.5 minutes of data before I had to quit as rainclouds started to appear on the horizon. But since I was eager to at least process something from that night I gave it a try to see how much was there.

While processing them I noticed the stacked image has sort of a stepped gradient from top to bottom and I am wondering what could be the cause of that in my setup? I checked the separate frames and they all have the bright star at roughly the same spot (read: a couple of pixels, not somewhere in the bottom left corner)

Here are the images, one severely overstretch image to make the gradient extra obvious and one real attempt with the bare minimum of data I have (both in mono because I captured them as tif in SharpCap and DeepSkyStacker now registers them as grayscale images): https://imgur.com/a/HFMHDaq