Something snapped in Japan today.

With Asian stocks finally breaking out a decade-long doldrum, and hitting record highs earlier in the session, and with Japanese equities starting off the session on the right foot and continuing their recent ascent which until Wednesday had seen them rise on 23 of the past 25 days, Japanese shares suddenly lurched on Thursday, plunging sharply lower after dramatic intraday swings took the Nikkei and Topix indexes to multi-decade highs only to drop in the afternoon on futures-driven trading ahead of the following day’s options settlement. All told, in a little over an hour, what had been another solid rally in Japanese stocks turned into some rather sharp clear-air turbulence, with the Nikkei 225 Stock Average plunging about 3.6% from the afternoon-session high to its low for the day.

It all started off well enough: in the morning session, the Topix notched a new 26-year high and the Nikkei 225 broke the 23,000 level for the first time since January 1992, as financial and securities shares rallied.

Then something flipped and in a gut-churning rollercoaster of a move, the Nikkei lurched from an over 2% gain which took it to a fresh 25 year high at the end of the morning session, to a loss of as much as 1.7%. The sudden reversal quickly spread to the currency market, with the yen surging before spreading across Asia: South Korean and Hong Kong equities also tumbled in sympathy. As Bloomberg snarks, “Sydney traders could count themselves lucky their market had already closed before the worst of the sell-off.”

Some traders laid the blame on technical factors – after all, the Nikkei has been on a tear, ending the day lower in just five times since the start of October (assuming Wednesday’s 0.04% “drop” was as unchanged). Others, like Bloomberg, wondered if the Nikkei had suffered a flashback: in an odd coincidence the Nikkei 225’s was its biggest reversal (points-wise) since exactly a year ago, when Donald Trump’s shock U.S. election victory rocked markets around the world… before they bounced back the next day.