metate arch

Stargazing at Metate Arch, Devils Garden, Grand Staircase-Escalante National Monument. Time-lapse over about 2 ½ hours, set up on a slider with pan/tilt and a 12mm lens.
Had the pleasure of meeting David Kingham and Jennifer Renwick on this night when they arrived with their workshop to photograph the same spot. I was about halfway through shooting this time-lapse and they were very classy and accommodating, shooting a couple other spots first to avoid having headlamps disturb my shooting of this scene. I appreciated their courtesy (and modeling this for their workshop participants), and it was great chatting with them while enjoying this amazing starscape.
Question: stars toward the edges are clearly starting to trail and and also look softer. I expected this to some degree at 25 seconds, and because I purposely set up the panning motion direction to make it look like the stars were moving faster (pan motion opposite of their movement in the sky). The Laowa 12mm lens is a bit soft at the edges, but any other thoughts on why the stars toward the edge look as soft as they do?
EXIF: sequence of about 300 images, 30 fps, each 25 sec, f/2.8, 12mm, ISO 3200, Sony a7s, Laowa 12mm 2.8. Kessler TLS bundle for motion control.