The Giants were down 5-1 in the 7th inning. However, Joe Panik singled to left to get that first run in the previous at bat, and there were two on and no outs.
Belt came up and watched a low pitch be called a third strike. It was a bad call from the home plate umpire, but as many fans will point out, with two strikes, you need to protect.
This is enigma that is Brandon Belt and his relationship with the strike zone. Belt has a strong eye and that eye leads to many walks over the course of a season.