And what does network hash rate follow?
Network hash rate follows the amount of processing power dedicated to mining, period. Since the peak in June there have been a number of different factors which would cause people to stop mining and subsequently cause the hash rate to drop off:
1. New content releases in WoW
2. Diablo III beta opens
3. Exchange rate for Bitcoins dropping
4. Battlefield 3 & Modern Warfare 3
5. Peak of summer when electric bills are highest
The list goes on, but nobody can (with a straight face anyway) lay claim to any single one of those factors as being the one that causes the network hash rate to drop.