One of the wildest runs in U.S. stock-market history began with the collapse of arcane bets on volatility and ended with a sober realization: The easy ride is over. After heart-stopping swings in the Dow (Down 1,000 points! Up 500 points!), a market correction is finally here.

But why — and why now? Inflation, interest rates, valuations, computers, ETFs, Trump; plenty of reasons were offered up. Among the lingering questions is the big one: Is this a hiccup or the start of something worse?

Monday, Feb. 5