I suspect, without any hard evidence at all, that exchanges are re-hypothecating their Bitcoin holdings. That is, they have less Bitcoin than their customers have purchased in their accounts. When supply gets squeezed, and people move their coin out and there is a bank exchange run, they will be caught with their pants down and it will likely cause them to collapse. There are other reasons an exchange can collapse: Regulatory, security breach, financial insolvency, operational failure, and fraud - including selling what you do not have. So which of the big exchanges do you think will be the next to collapse?