The Chiefs picked up RT Eric Winston in free agency which would seemingly eliminate the need for an offensive tackle in the draft. But KC is surprised to see Reiff drop this far. GM Scott Pioli has a good relationship with Iowa's Kirk Ferentz and they don't pass up an opportunity to select Reiff.
SB Nation's Chiefs blog, Arrowhead Pride, explains:
The Chiefs are stunned when they see Reiff, considered by many to be a better prospect than Mike Adams (No. 10 to Buffalo), still available with the 11th pick. This is a best player available type of situation and the Chiefs select Reiff knowing full well they have two NFL starters already on the offensive line.
Reiff is the future replacement for LT Branden Albert, who is entering the final year of his contract. Reiff's availability means the Chiefs don't have to give Albert a long-term deal and could entertain trading him. Reiff comes from Iowa, where he was coached by Kirk Ferentz, a close friend of Chiefs GM Scott Pioli. Iowa linemen are noted for how well-coached they are and Reiff is no different. He pushes for playing time immediately but he's not truly in the plans until 2013.
Riley Reiff 2012 NFL Draft Profile (via sbnnfl)