I've been compiling a list of these things, have not finished quite yet though.

There have been 2 flash crashes this year so far, the other being on 3rd January at 00:15 affecting both CHF and JPY pairs.

The last time this happened (to my knowledge) was 2016 on GBPUSD

All the drops were significant 4-7% depending on the pair

All times the moved happened very quickly (5 minutes max)

All times the price recovered most of the losses within an hour

All happened at times of low liquidity (just past midnight for the first two, market open for the last one)

Both times this year there was a Holiday in Japan

In each case, no one knows what event started it (though everyone has an opinion).

I would say. If you were planning a combined attack on a currency pair, you certainly would time it for times of low liquidity. Even better if there's also a public holiday.

Westie removes his tin foil hat ;)

Edit :-

Though this sounds far fetched, I've noticed a huge increase in so called 'Momentum' EA's and signals in the past 5 years. So it could be possible, with enough combined capital, at the right time of day, to push price fast enough to trigger all the Momentum EA's - multiplying your initial attack dramatically with no cost to yourself.

Will look forward to seeing what the FCA / BIS etc find when they've had a chance to look through all the data.