Why do some airborne ADS-B traffic targets display as diamonds instead of arrowheads?

ADS-B traffic broadcasts will not always have heading information for all aircraft.  Heading information is required in order for ForeFlight Mobile to present a traffic icon as an arrowhead.  When heading information is missing, the traffic icon will be shown as a diamond.   

Image 1.  Airborne traffic icon showing as a diamond due to the lack of heading information for that aircraft.