For some men, male pattern baldness is sudden and catastrophic, for some slow and limited.
In my view, Propecia/finasteride is not the cause of shedding, finasteride has a very slow, postitive effect on DHT etc and the approx 5 year hair growth cycle. You posted here because you have noticed a coincidental relationship between the finasteride and the shed. It can only be a coincidence.
Having lost practically all my "Norwood" area hair, I started from shiny scratch, and because now I maybe have only several thousand noticible hairs, either strong vellus or long terminal, the normal percentage loss will obviously be relatively small. In a few more years, when I fully expect to have a larger number of terminal hairs in proportion to strong vellus, my sheds will probably be noticible. At present I notife just one or two strong vellus hairs loose on my forehead after brushing in the emu oil (after the saw palmetto, minoxidil & retin A).
I hope you are a good responder to finasteride, but you need to do all the other good therapies on a daily basis.
Boru