But what causes the price to rise?
Speculators, people who think it's easy money? Who knows. There may also be an element of good old fashioned "pump and dump".
There have been quite a few Bitcoin thefts, and given the nature of the currency, we probably don't hear about them all. There's also the tale, possibly urban myth, of the bloke who threw away his faulty hard drive, and with it, a small fortune in Bitcoin.