There is no direct way of finding out why other than investingating...How far did your
PR fall?
PR can fall as the number of pages on the web increase, so this is normal. If you had a drastic decrease it could be something else. Typically Google will rank a penalized site 0 not just decrease by 1.
This is a good resource for investigating further...
http://www.webmasterworld.com/forum5/4584.htm