The reasoning for this pick is quite simple:
The Pistons' roster needs a massive overhaul. Walker's a brilliant piece to build around.
Many expect Detroit to take a big man, especially if Bismack Biyombo is still on the board. He is available in this mock draft, but with Walker there it'd be tough to pass up on another former UConn player. He would be a player for the Pistons to build around, as the quote above says, and he would give them the ability to not play Rodney Stuckey at point guard.
Here's a look at the first seven picks of SB Nation's mock draft: Kyrie Irving (Cavaliers), Derrick Williams (Timberwolves), Enes Kanter (Jazz), Jonas Valanciunas (Cavaliers), Jan Vesely (Raptors), Kawhi Leonard (Wizards) and Brandon Knight (Kings). Marcus Morris (Bobcats) and Klay Thompson (Bucks) round out the top 10.