Minoxidil's mechanism (though not completely understood) is much more likely to cause a shed. Minoxidil's manufacturers even warn of a shed with their product. The mechanism by which Propecia functions shouldn't really cause a shed. There's no "shock" to the follicles, just a fairly gradual lessening of those little DHT boogers in the scalp. Using both in conjunction would probably cause an even larger shed, as more follicles are then susceptible to the shock (i.e. more follicles are recovering from DHT and therefore ready to enter a growing phase). It seems that many people are probably going through a regular male pattern baldness shed and happen to go one of the drugs at the same time, which would give every appearance that the drug initiated the shed (while, in actuality, it was just a function of your condition).