"The GBP/USD slide could be due to erroneous order and/or flows related to stop-loss orders or options given USD/JPY or EUR/USD aren’t moving much", says Toshihiko Sakai, Tokyo-based chief manager of FX and financial products trading at Mitsubishi UFJ Trust & Banking.

"Looks like there was a large GBP sell order amid thin liquidity", says Kyosuke Suzuki, head of FX and money-market sales at Societe Generale.

Others believe that the massive move has been partly attributed to algos failing after traders targeted downside option barriers, say three Asia-based FX dealers. Traders typically place their nearest orders within 100 ticks of spot, which was at roughly 1.26 before today’s plunge.

The drop accelerated as liquidity disappeared, and dealers failed to load bids into their trading platforms, say traders. In other words, your plain, garden variety algo-facilitated flash crash, where the bid side suddenly disappears as one or more "liquidity providers" turn themselves off.

Another question: whether any FX brokerages will need a bailout a la the infamous FXCM, in the aftermath of the Swiss National Bank revaluation of January 2015, as clients find themselves margined out and underwater even as cable is steadily recovering most, if not all losses.

Algos, Barriers, Rumors: Some Theories On What Caused The Pound Flash Crash