Other mistake: When Kenneth and Andy exchange via white board about Fort Pastor, you can see a guy chilling on the roof of a building nearby in the background just over Andy's head when he ask " so what's the bad news." He is not trying to get the attention of either Kenneth or Andy. (00:47:28)
Other mistake: Near the beginning, Ana speeds away from her home down winding streets in the suburbs. In an aerial views of the houses, look closely at the row of homes at the bottom of the screen. None of the homes are shown with driveways or sidewalks. Essentially, there is no way getting to these homes. This is revealed in the commentary by the director who realized the mistake, but didn't care.
Answer: It was actually explained in both versions that the zombies went to the mall because it was a place of familiarity, a place that they could remember even if they didn't know why.
Tobin OReilly