Your problem is not so rare, in fact it is very common in the newer tighter homes.
You call it back drafting , we call it draft reversal ,
this is casued when a natural draft vent originates in a low pressure area of the home and terminates in to a higher pressure area.

as we tighten up the side wall construction of homes it prevents a adequate amount of air from entering the interior enviroment.
at the same time in the higher regions of the home there is an equal amount of air exiting the home which contributes to the negitive pressure of the enviroment.
this presure difference can also be affected by any exhaust fan that does not have a make up air supply. every house has a number of these, bathroom exhaust fans, clothes dryers, range hoods ext.
another source of out going air is any non air tight penitration in the higher level ceiling suche as recessed lighting an the bigest one is the attic access
the only solution is to provide for and equal and slightly larger amount of air supply to the area of the natural draft device.