As mentioned, margin call, or, maybe even more likely, plain old fashioned belief in future price increase. I'm firmly in the bear segment for now, but even I can see how it might make sense to buy in now with a large sum (at the risk of short-term losing some of your depot's value) under the assumption that the long term growth potential that outweighs that risk is still there. It's all about different time frames in which market participants calculate (and, no, I don't necessarily say those who think "long term" are making the smarter choice).