To fix the Fix, bullion bank traders (whether they are direct participants or not of the fix process) have to be able to “buy the fix and sell the futures” when the fix gets swamped by sell orders (or vice versa). The problem is that banks have a wide range of clients who hold positions with them across spot, forwards, futures, options, ETFs and so on. It is therefore highly like that one of those clients would be the loser of any such activity (and others winners) and complain (as did the client Barclays’ trader Daniel Plunkett traded against) about it. Alternatively, the regulator may decide to investigate markets from time to time.
The problem is that when a regulator comes looking at trades after
the fact they could construe manipulative intent when no such thought
was going through the trader’s mind – who was just arbitraging a market
imbalance – and the trader finds themselves fined £95,600 and banned
from trading, as Plunkett was.
If you think that traders would not be worried about such an unfair
claim against them happening, or that client complaints or random
regulatory investigations it would be unlikely, you haven’t been reading
enough Matt Levine
Read more here