From AvWeb:
In the early 1970s, general aviation switched to dry vacuum pumps manufactured principally by Airborne and relative newcomer Sigma-Tek. Although there are differences between the two, Airborne and Sigma-Tek pumps use self-lubricating graphite vanes which are both the solution (no engine oil required for lubrication) and the problem (the pumps tend to produce spec vacuum throughout their lives and then disintegrate internally in a cloud of carbon dust).
Compounding the sudden failure mode is a frangible coupling that is designed to shear abruptly in the event of overstress or sudden stoppage. In most cases, the aircraft operator will see no sign of any problem until the vacuum gage reading falls precipitously to zero.
Perhaps backwards pulling of the prop causes the vanes to dig in, and refer to the latter para ...
however a further search of AVWeb got me:
http://www.avweb.com/news/maint/182905-1.html
Backwards is Bad
The hub slots of Airborne pumps are canted in the direction of rotation. For this reason, Airborne offers different pump models for clockwise and counterclockwise applications. The most common model numbers are 211CC and 441CC (for counterclockwise rotation) and 212CW and 442CW (for clockwise rotation). It's not difficult to break the code.
Installing a wrong-direction pump is a sure prescription for premature failure. Most Continental engines require a clockwise pump, and most Lycomings require a counterclockwise pump. But not always. In fact, twins with counter- rotating props need one of each!
so it would appear that turning one backwards WILL cause problems depending on it's installation.