They are not "star trails" Peter. That is camera movement. The trail are not straight (for that short an exposure, longer exposures they are curved), they are far too long for 15sec. and there is a "bump" in the middle. I would guess a gust of wind, tripod movement or someone walking near the tripod during the exposure.
If I had to take one guess I would say the camera was moving downward under the force of gravity somehow. Either the tripod was settling, the camera mount wasn't tight of someone stepped near a leg particularly if it was on grass or a timber surface, but can happen on concrete as well but not usually at that FL unless its "wobbly" concrete
