You may have noticed stocks have stopped going straight up. By itself, that's not scary. Bull markets and high volatility can coexist, and often did until the last few years.

No, the scary part is that bull markets fall apart slowly.

Even in 2008, the bull took a long time to collapse. The Bear Stearns collapse in March was even followed by a nice rally because people thought the worst was over. But six months later Lehman went bankrupt, and then it was six more months before the final bottom in March 2009.Smaller cracks accumulate to set up the big breaks. History shows 2008 was a terrible year—but the Dow had peaked months earlier, in October 2007.

So, like the metaphorical frog in boiling water, you can be in the middle of a bull market's unraveling and not even know it—or at least not know how bad it will get or when it will end.

That should worry you because right now, the number of cracks is growing while the ability to patch them up is not. Let's look at just three of those cracks.

Photo: Getty Images

Crack #1: Cryptocurrencies

Last fall, investors went crazy for crypto, mainly Bitcoin. Prices climbed fast as “fear of missing out” caused near-panic buying in some quarters.

Panic buying is almost always a mistake, and sure enough, bitcoin fell from its peak of $19,000 in December to below $7,000 today.

Bitcoin fans are undeterred and convinced it will be back. Maybe so. But the more immediate effect is that crypto-mania soaked up a lot of uninformed risk capital that is now less able—and probably less willing—to take the risk of buying stocks.

This is important because according to Dow Theory, “distribution” is the first phase of a primary downtrend. That's when the last buyers enter the market, buying stocks from big sellers who perceive signs of trouble. It gets worse from there… eventually a lot worse.

In this case, the distribution phase could already be near its end if those last few buyers already spent (and often lost) their cash on other, non-equity assets like cryptocurrencies.