Just one question though: who have they bought those derivatives from? Why, other banks of course. This creates what is known as counterparty risk. Bank A sells insurance to Bank B. But then Bank A gets into financial difficulties (a significant deterioration in their creditworthiness would be enough) and suddenly Bank B isn’t as well protected as it thought it was.
Indeed, Bank A might start struggling precisely because of the insurance it has sold to Bank B. What if it can’t honour the contract? This creates a potential Catch-22 situation: the derivatives work as long as they’re not needed; calling them into action renders them useless.
This is precisely the kind of thing that occurred during the credit crunch - banks stopped trusting each other. New rules introduced since then require banks to actively manage their counterparty risk. In other words, banks are being asked to hedge their hedges.