From the still images as Jewett made the tight pass and got to the rail his back and shoulders were arched indicating deceleration . Look for that . That puts Amos who in kick mentality and likely ready to accelerate in a bad position.
However , If there was no violation on passing , no cutting off Amos, why rule " incidental" contact? If it happened on a freeway and the car in front had no fault the car behind would one hundred percent be the liable party.
I don't agree that the contact was " incidental" at all , they were running in a single file once the pass was made. Either the pass caused the contact ,or Amos caused it, or both did, but it wasn't "incidental". Amos was just starting to go around Jewett, presumably at a faster speed than Jewett himself was running. This does put some scrutiny on him. He could have taken a few inches of Jewett's back kick space with the new angle and by not slightly decelerating , or putting a small hitch in his stride to pass, as is the norm in tight conditions.