Rehypothecation has been put forward for the reason of the massive sell off of gold over the last few days.This can be defined as follows:

Rehypothecation is a practice that occurs principally in the financial markets, where a bank or other broker-dealer reuses the collateral pledged by its clients as collateral for its own borrowing.”

So in relation to GLD (which is owned by HSBC), an article by ZeroHedge has suggested that after MF Global, investors are beginning to suspect what very few knew already, was that gold accounts owned by clients had their gold reused  and relent. So fewer people are trusting paper gold or that GLD will be of much use when SHTF.

Source: ZeroHedge


A good interview below by Kyle Bass also touches this. Fast forward to relevent part on collateral at 9min 30 secs in.