Tapering the back reduces turbulence as the air moves from around the object to rejoin itself in the back. Turbulence lowers pressure behind the object, sucking it backwards.
Curiously, a station wagon with a flat rear end is no less aerodynamic than an equivalent sedan, even though the latter is more tapered. Everyone laughed at Volvo because when the entered the BTCC (British Touring Car Championship) in 1994, because they decided to race the 850 estate/station wagon rather than the saloon/sedan.
Then Volvo pointed out that the difference in drag was essentially zero, but the estate produced significantly more downforce, and everybody stopped laughing.
The same can be seen with hull shapes: wider at the bows and tapered at the stern.
The teardrop car: http://www.antiquecarsclub.com/need-a-new-old-car-66.html