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I think that Nik is on the money by identifying tracking as a likely suspect. Looking closely at the cropped image with the bright star in the centre, you can see that all the stars in the image have a horizontal elongation to them. This is indicative of a tracking/guiding issue (assuming all subs are affected the same) or if this is a single sub it could be that the scope was bumped during the exposure.
Looking closely at the bright star in the centre of the frame, I can see that it is extended a small amount to the right which indicates something has shifted during the exposure.
It might be worthwhile as well to check the polar alignment. If you are using PHD then try running the guiding assistant which will report on polar alignment error and the RA peak to peak performance of the mount.
And lastly, check that there are no cables snagging as the mount is tracking.
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