Why does this happen?

[alfrank]

Specifically, why does the line of nodes (periapsis and apoapsis) rotate so that apo and peri have switched places I find this occurs when I'm trying to circularize an orbit and it's confusing me. 

[Superfluous J]

If your orbit is a circle, the Ap and Pe are basically the same so even a tiny change can move them around. When they do that, you can just ignore them because all they're telling you is that they're both about equal to your average orbit height.

[Geonovast]

AP is the highest point in your orbit.
PE is the lowest.

If you're at AP and raising your PE to circularize, and the PE gets higher than the AP (even if it's just a tiny fraction of a meter), then it's no longer the lowest point.  It's now the highest, and your AP becomes the lowest, so they switch.

[darthgently]

12 hours ago, Superfluous J said:

If your orbit is a circle, the Ap and Pe are basically the same so even a tiny change can move them around. When they do that, you can just ignore them because all they're telling you is that they're both about equal to your average orbit height.

I've often wished that when eccentricity got so low that AP and PE just jitter that the UI would just hide them.  Maybe the orbit line could shift in hue a bit to indicate a pretty much circular orbit so the apsis disappearing isn't confusing