There are three reasonably likely causes for star trails with "bumps" like the ones shown:
First is camera movement during the exposure. If the tripod wasn't quite steady enough, or something (a cat or dog, for instance) rubbed against one of the legs, you could get a "bump" in the trail. What shows here goes on for much of the exposure, so it would be a very persistent cat, if that's the answer. Minor earthquakes could do this, too, if you're in an area prone to those -- but again, they'd tend to be momentary, so affect only a very small segment of each star trail.
Second is air movement -- what astronomers call the "steadiness" component of "seeing" -- what we see as stars twinkling is actually movement of their images due to variations in density of the air changing the path of the light. This will tend to be more or less random, and is likely to be worse early in the night and get better as the air mass cools and turbulence drops off -- though sometimes, seeing will remain bad all night.
Third is what I learned to call "jaggies" when I was doing computer programming back in the early days of computer graphics (ca. 1980). This is an artifact of digital displays and sensors and of the quantized nature of the coordinate systems they use -- and doesn't appear applicable to the random-seeming bumps in your trails.
Based on what I see, I believe your issue is due to air movement, aka "seeing".
What you can do about it? There are web sites that provide seeing reports (often including dark sky and air clarity reports); they're aimed at deep sky observers and photography, but seeing is important to any long exposure astrophography. The first thing is: don't expect excellent results when the seeing is bad. Things will tend to be worst just after dark, when roofs and pavement are cooling from the day's solar heating, and when there's wind (especially if it's gusty). Wind will also contribute to tripod movement.
Can you fix this in post? To some level, by applying blurring and sharpening it might be possible to smooth out the trails -- but you'll never get the classic, dead smooth trails you'd see from a really steady mount with good seeing.