I think it was the fact that he extended his arms with what appeared to be a push even if soft push. If he just collided with him because they were running full speed and just crashed i bet they might not have called it.IDK. I thought it was a very soft call. Incidental contact and Mahomes flopped. But the CBS rules expert disagreed so what do I know.
Also, seemed like a big holding call before mahomes started running which could have negated the entire thing anyway.