I've seen this several times. The miners and the small merchants who sell in bitcoin will wait till the end of the month when lots of expenses come due. Then they all sell. Then the price drops. Then when this happens, the many speculators and tradebots sell their coins and it spirals down.
I just looked at a 6 month chart from Mt Gox. If my link works, it is
http://bitcoincharts.com/charts/mtgoxUSD#rg180ztgCzm1g10zm2g25.
End April/1st week of May: Slight dip then gradual uptrend
End May/1st week of June: Steep uptrend then steeper uptrend
End June/1st week of July: Steep dip, then slight dip
End Julyl/1st week of August: Slight dip, then steep dip
End August/1st week of September: Not quite there yet
Form your own conclusion.