The suspicion of such collusion – as suggested by some emails and chat-room discussions that have come to light – is being taken very seriously. The existence of collusion cannot be inferred, though, from the volume of trading during the fixing window.
Banks may want to execute many of their own trades during the fixing window, for instance, because their customers want to transact at the fixing price, and banks therefore look to offset those positions around the fix so as to manage their market risk.