I realize this is a rather old post but the store was used as a military safezone for survivors. Someone in the zone may have died (I'm assuming this is the case because Sasha mentions luring them with music after cutting a hole in the fence) and began infecting others. The helicopter could have been attempting to take off with individuals on the helicopter and crashed. There is nothing to suggest, however, that there are not simply stairs leading up to the roof of the building and the zombies went up to check out the noise made by the crash; all it would take is one stumbling into the door trapping them up there.