You can check out the usual suspects..
From your image what was the direction of the trail? In RA or DEC??
You can use your planetarium program to determine the Alt/Az at the time you took the image and see if the track follows Newton's gravity.
It could be cable snagging?
Set up the guide on an artificial star - if you don't want to waste observing time and apply hand pressure to the guide scope, the focuser, the extension tubes and the camera and watch for any movement in the guide star image..
(GSE is good for this sort of Stuff!)
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