That's weird indeed.
You can eliminate a few things though. It's not your guiding and your camera/focuser is square. What's left is the optics. It doesn't look like it's coma. I've never see coma only on opposite corners and not the others. I have no idea what it could be. Astigmatism maybe? But then again it should show in other parts of the picture and pretty much all your stars are in focus. Something's not straight somewhere but hard to tell. Slight mirror misalignment maybe? (as in optical axis not matching)