Cam
Remember those are 3 of 60 subs at different times in the sequence and the 4th image is a different target. Next clear night I'll go out and try again and see if it happens in the same way. If it doesn't then it must be cloud. If it does it could be glow from the sensor as Kevin suggested.
Tony
What adjustments did you do in photoshop? I don't have it but if it can improve an image that much it could be worth getting.