Originally Posted by Sunny Day

Since I'm so dumb, I'll have to explain it in simple terms. Hopefully your smart brain can follow.

I buy 1BC from MtGox for $100 + fees. MtGox then holds my BC. In the mean time, on other exchanges BC has risen to $150., but MtGox hasn't sold any more BCs, so they only have $100 in reserve. I decide to sell my 1BC for $150. MtGox can't cover the sale.

Simplistic view, but when you multiply that by a fast rising commodity and lots of sellers wanting to cash out, that what happens.