Who do you have in mind? I don't see anyone who wants Bitcoin to crash desperately enough to spend millions on inflating a bubble and then pop it - which is a very risky and costly plan. More likley would be a scenario in which someone who holds a lot of Bitcoins pushes MtGox prices upwards in order to be able to sell OTC at inflated prices. However, given that Bitcoin on MtGox has a volume of USD 20 million daily, this takes a lot of resources. It is much easier to manipulate a low-volume stock on the stock market.