Forex market is very huge and retail traders are on very small scale if we talk about volumes. Big players like banks and financial institutions make big moves by making trades with higher lots and that shakes the volatility.
Some points here seem valid, others not. It's true that a large portion of the FX liquidity is controlled by around 4-5 banks as mentioned (Citi, Deutsche, JP Morgan, UBS). These are the banks with the most connections to corporations, transacting with them for international trade and foreign currency conversions.
It's true that the big banks have a better visibility on order flow, but it's not really true that they monitor the retail positions in the market (As this is a very tiny portion of what's happening in the market). The banks will rarely take directional positions as they seek to profit from matching opposing order flows (Acting primarily as liquidity providers or market makers), thus profiting from the spreads.
On top of the spot FX desks, the derivatives desks in the banks create custom made structures (Using FX options, swaps and swaptions), sometimes worth hundreds of millions of dollars & sell them to corporates. Those structures act like selling naked or covered options, but they are much more complex as they might involve many legs spanning several months in the future and cannot really be exited as there is no market for them except to close them out with your counterparty. This is where the manipulation of the WM/Reuters fixing comes into place, as this WM/Reuters fixing is the basis for valuing those structures and determining if the structure would be a profit or a loss for the bank (And the corporate who bought that structure), hence the attempt to manipulate it as it makes or breaks billions of dollars in structured products the banks have in place.