The stock bull market finds itself in crisis mode as it turns 11 years old. Here are the 4 reasons why it's lasted so long - and why the end could be near.

Corporate earnings growth has long been one of the most important drivers of market returns, and has been fundamental to the record bull run.

"Markets rise and fall on two things: earnings and valuations. If earnings go up (other things being equal), so should stock prices," Brad McMillan, chief investment officer at Commonwealth Financial Network, said in a Friday note.

Profits have grown in 10 of 16 quarters since 2016. That's also supported valuations, as the ratio of price-to-earnings (P/E) is a popular way to measure company worth. When profits are growing, the denominator in the equation rises, which can shrink or control P/E and encourage investors to buy.

Companies have used buybacks as a way to boost stock prices for shareholders, providing an instant jolt to price by shrinking the pool of outstanding shares.

Buybacks kept the market on track from 2015 through mid-2016, when S&P 500 earnings growth shrank for five straight quarters in a row. The Tax Cuts and Jobs Act in 2017 also gave stock repurchases a hand by making it easier for companies to bring cash stored overseas back to the US.

That sent buybacks soaring to a record $1.1 trillion in 2018. Since, the pace of buybacks has slowed slightly, and they are projected to hit $1 trillion in 2019.

The Federal Reserve set interest rates near zero in the wake of the financial crisis, and they have stayed historically low ever since. When rates are low, it makes money cheap for businesses, which use it to make acquisitions, repurchase their own shares, and invest in research and development to continue to grow — all things that drive future earnings and stock market returns.

Low rates look poised to continue. On Tuesday, the Federal Reserve handed down a half-percentage point emergency interest rate cut amid market volatility due to the coronavirus outbreak.

As the longest market expansion ever has continued on, investors have gotten used to stocks going higher. Those who bought when stocks did dip slightly have been rewarded time and time again, which in turn boosts investor sentiment and participation in the stock market.

It's now become an often-heard market phrase. On Friday, Larry Kuldow, National Economic Council Director and President Trump's top economic adviser told CNBC that investors should "think seriously about buying these dips," referring to the current market downturn.

The coronavirus outbreak has sent markets into a tailspin. The outbreak overshadowed an otherwise solid fourth quarter 2019 earnings season, and will further weigh on companies going forward — some including Apple and Microsoft have warned that they might not hit future earnings guidance numbers or have even withdrawn estimates in light of the outbreak. Goldman Sachs now estimates that US companies will post zero earnings growth in 2020 because of coronavirus.

In addition, the US economy is likely to take a hit as the coronavirus disrupts supply chains, keeps workers at home, and potentially hinders consumer confidence. While the Federal Reserve issued an emergency rate cut to spur the US economy, markets haven't reacted positively — stocks have continued to fall while long-term US Treasury bonds rally, showing that traders are still banking on further cuts from the central bank.

If markets continue to fall, it could mark the end of the bull market. It's possible that the market could fall into bear territory — a drop of 20% or more from the recent high — is according to Lori Calvasina, head of US equity strategy at RBC. That could send the US into a recession.