MtGox not allowing USD withdrawals
Yes, it might be the start
I think it's the main cause because most BTC<--->USD (or other fiat currencies) trades went through Mt. Gox. Now that people can't withdraw fiat, the demand starts to rise while the supply doesn't cover it. Thus the price rises. Such rise is noticed by people on other exchanges whose main intention is to sell high so the bitcoin price rises at least a bit in all exchanges.