I've been fed up of seeing reasons that may cause price drop. So I'm trying to list them out. Please add your reason...
1. China banned Bitcoin (worked for 3-4 time)
2. Ghash.io is nearing 51% (worked twice)
3. Mt. Gox fall is fall of Bitcoin (upcoming ones => Bitstamp is insolvent & BTC-e is run by anonymous people)
I'll try to update the list occasionally...
Latest Update:
Point 3 added