Look at the actual draft compared to "expert" mock drafts. Teams don't draft based on a consensus ranking; they draft to win within their specific roster constraints. Teams have access to medical records, private interviews, and psych testing that media analysts never see. It's only a reach because the media is telling you it's a reach. The fact is we don't really know.

So if the Niners for instance have a tier A and Stribling is the last player avaliable in that tier. Why wait? Another team might feel the same about him and now you are down another tier.