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I see that often, too, especially with aircraft tails. I suspect it's to do with the change in angle, relative to the moving object. The slower your shutter speed, the more time the object has to move and get out of focus/motion blurred.
Not sure how to combat it really other than use a faster speed or just concentrate on the driver or pilot and get them sharp, at least. Anyone else...?
Yes, Jo is in the right of it: simple geometry is working against you at the slower shutter speeds. When you pan with an subject moving across in front of you, various bits of it are moving at slightly different rates and directions through your frame even when the subject, as a whole, is tracked perfectly. So at the slower speeds you can expect only some part of the subject to be sharp, no matter how good your technique.