The shared bright–dim pattern usually means something affected the whole frame at once, not the stars individually. Based on the answers, the most likely causes are:

• changing exposure during a composite/stacked sequence, especially if the photographer adjusted for brighter events like stage separation or boost-back burn
• varying atmospheric clarity, such as thin or fast-moving clouds dimming all stars at the same time
• later processing, such as black-level or gamma adjustments, which can make parts of trails fade or even appear to drop out

Because all trails show a similar pattern at the same point in time, it’s unlikely to be caused by the stars themselves. In a single long exposure, passing haze or clouds can dim the entire sky briefly. In a stacked image, one or more frames may have been exposed or processed differently, producing the same brightness change across every trail.

So the non-uniformity is most likely due to exposure/processing changes or changing sky transparency during the capture, not any unusual motion of the stars.