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What is the cause of sudden price spikes in the FOREX market?
It depends on the currency pair since it is much harder to move a liquid market like Fiber (EURUSD) or Cable (GBPUSD) than it is to move illiquid markets such as USDTRY, however, it will mostly be big banks and big hedge funds adjusting their positions or speculating (not just on the currency or market making but also speculating in foreign instruments). I once was involved in a one-off USD 56 million FX trade without which the hedge fund could not trade as its subscriptions were in a different currency to the fund currency. Although it was big by their standards it was small compared with the volumes we expected from other clients.  Governments and big companies who need to pay costs in a foreign currency or receive income in one will also do this but less frequently and will almost always do this through a nominated bank (in the case of large firms).  Because they need the foreign currency immediately; if you've ever tried to pay a bill in the US denominated in Dollars using Euros you'll know that they aren't widely accepted. So if I need to pay a large bill to a supplier in Dollars and all I have is Euros I may move the market. Similarly if I am trying to buy a large number of shares in a US company and all I have is Euros I'll lose the opportunity.