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.

• Monday marks the 11th anniversary of the longest running stock bull-market.
• Since the post-financial crisis low on March 9, 2009, the S&P 500 has returned 339% through Friday's close.
• Here are four main drivers of the market's bull run, and why it could soon come to an end.
• Read more on Business Insider.

March 9, 2020, marks the 11th anniversary of the longest running stock bull-market in history.

Stocks have been climbing higher since the post-financial crisis low on March 9, 2009. The bull market in equities officially became the longest-ever in August 2018, and has continued to gain since. A bull market is defined as a 20% increase on a closing basis that is never interrupted by a subsequent 20% decline.

The S&P 500 has gained 339% in the 11-year period through Friday's close and boasted an annualized return of 15.3% through 2019. While markets move in reaction to a number of events, there have been four main drivers of the most recent bull rally - strong corporate earnings, stock buybacks, easy monetary policy, and solid participation.

The record bull run also faces perhaps its biggest threat as the coronavirus outbreak spreads, sending markets into a tailspin as investors worry that global growth will grind to a halt. So far, the coronavirus that originated in Wuhan, China, has infected more than 100,000 people, killed more than 3,300, and spread to more than 88 countries including the US.

At the end of trading in New York on Friday, stocks were down 12% from the most recent high on February 19 as investors panic that the spread of the virus will halt global growth. If the market continues to fall, it would mean that the expansion has already ended.

"We don't know where this correction bottoms out," said Liz Ann Sonders, chief investment officer of Charles Schwab, in an interview with Markets Insider.

"And if the weakness that we're inevitably going to get is sufficient enough to go into a recession," it's more likely that the market turns into a bear market, even if it's not a severe one, Sonders said.

Here's a look at the four main drivers of the current record expansion, and why it might soon end.