Automatic Doors that Won't Open Automatically

It's a sticky thing.  Sometimes the automatic door at my post office isn't automatic. That is, you have to actually push the door open.

Strange thing is that people will often stand in front of the door, waiting for it to open.  They don't bother to try pushing it.  One time the person in front of me actually backed away from the door and allowed me to push it open.  Today, the person backed up and tried to walk towards the door again. Maybe this time it would open for them.

Why not just push it open?  Why hesitate, wait, or back away? What's up with that?

Maybe there could be a business idea here.