For the second time in four years, a would-be future speaker of the House lost a rather shocking primary to an unheralded challenger.

This time, it was Democrats’ turn.

But while then-House Majority Leader Eric Cantor’s 2014 primary loss fit neatly into the narrative of a Republican Party skeptical of compromise and looking to shake things up, what accounts for the defeat of the House’s fourth-ranking Democrat, Rep. Joe Crowley (D-N.Y.), on Tuesday?

Crowley’s loss to Alexandria Ocasio-Cortez wasn’t totally out of the blue. As The Washington Post’s Dave Weigel previewed this week, there was considerable interest in her challenge from the left of a member of the House leadership. But it wasn’t supposed to be this easy. The latest results show her leading Crowley by 15 points.

Like Cantor (R-Va.), Crowley was earmarked by some for the top spot in the House. With plenty of consternation about the party’s very old leaders, Crowley stood out as the not-quite-as-old alternative. The top three House Democrats — Nancy Pelosi, Steny Hoyer and James Clyburn — are all between 77 and 79 years old; Crowley is a relatively tender 56.

But on Tuesday, he lost to a challenger exactly half his age (28) — and one who would be the youngest member of Congress by five years if she were to join the House today. With growing resistance to Pelosi’s bid to return as speaker, it’s not hard to draw very cursory conclusions based upon just that fact — and many will (perhaps not wrongly).

Nor is it difficult to draw a conclusion based on Ocasio-Cortez’s more left-leaning politics. While being a member of leadership sometimes requires compromises that may not have endeared Crowley to the more liberal constituencies of his Queens- and Bronx-based New York district, Ocasio-Cortez is a member of the Democratic Socialists of America and campaigned on universal Medicare and abolishing U.S. Immigration and Customs Enforcement (ICE).

But neither explanation is so simple. While Pelosi has earned her detractors in the House Democratic Caucus, they’re largely members worried about losing rather than members worried about her age. She has become a rather convenient and effective punching bag for the GOP, and a growing number of House Democrats who appreciate her leadership (and fundraising) are simply ready to cut bait in the name of winning back the House.

As for the second theory: It’s not like the left is lighting the world on fire in primaries this year. While the Bernie Sanders-ification of the Democratic Party has proceeded apace, Crowley comes from a remarkably liberal district (Hillary Clinton won it by 58 points in 2016) and is the first Democratic incumbent to lose a primary to a more liberal challenger this year. And that’s with the majority of primary races already in the books for 2018. If this was a canary in the Democratic Party’s coal mine, someone forgot to tell basically every other primary electorate that the party was ready to throw the bums out in favor of real liberals.

And the leftward lean of the district wasn't the only factor conducive to a primary upset: So was its demographic makeup. Crowley represents a district that is less than one-quarter white and nearly half (48 percent) Latino. And in an open primary without an incumbent, a Latino candidate facing a white candidate would be expected to win.

Of course, even when Cantor was losing, it wasn’t as if a ton of GOP incumbents went down with him. Just two lost that year in states whose maps hadn’t been redrawn, in fact. But looking back, it’s not difficult to say that Cantor’s loss was an expression of dissatisfaction with the party and its leaders. Just a year later, after all, Donald Trump was leading the GOP presidential primary contest, and the party has remade itself in his image since then. The years-long fuse that was the tea party eventually hit Cantor’s district and then detonated with Trump.

That’s not to say the Democratic Party is headed down a similar path or that Crowley didn’t make mistakes. Cantor certainly did. An incumbent has to make mistakes to be vulnerable to a primary defeat, because it’s so difficult otherwise for challengers to get any traction. That’s a big reason we don’t see more incumbent primary losses, even when there is legitimate unrest, as in the GOP’s tea party years.

But if Cantor's loss was yet another isolated sign of unrest in the Republican Party in 2014, Crowley's defeat is perhaps an early signal of unrest in the Democratic base in 2018 — a sign of exasperation that the old way isn’t quite getting it done. Or maybe it’s just a freak set of circumstances. The point is that Democratic leaders have to ask themselves which it is in a way they simply didn’t just 24 hours ago. That’s the lesson of Crowley’s loss.