I think it's falling because it will be deleted from the Russell 3000 index at end June 2011. The Russell indices are 'reconstituted' once a year at the end of every June. Russell published the proposed 2011 additions and deletions to their funds on 10 June - NuPathe was on the deletions list. In addition, traders can second guess the likely additions and deletions a couple of weeks in advance as Russell publish the technical rules behind their reconstitution technique.
Deletion from the Russell index is a real technical hit - index trackers have to unload it & many institutional fund mandates require stocks to be on Russell indices. I suspect the price will drift down 'til end June - probably with a nasty fall around the reconstitution date.
In my view, this would be a perfect buying opportunity for the upcoming NDA catalyst.