This is why Apple stock was down over 2% Thursday

As markets broadly weakened Thursday, one stock in particular stood out: Apple. The tech giant's shares turned negative in afternoon trading amid a spike in volume and closed down 2.04 percent to $120.07. Trading volume was 78 million shares, almost double the average of 40 million. (Tweet this)

So what happened?

UBS might have something to do with it. The bank released a note Thursday pointing out that greater China has provided over half of Apple's recent growth in revenue.

So a slowing Chinese economy could be a real risk for the company. "We think Apple is OK for now, but we will watch the situation closely," UBS economic Tao Wang wrote.

Read More Apple Watch online sales plunge 90% in US: Report

According to FactSet, China makes up about 16.2 percent of Apple's total revenue.

The Chinese stock market rebounded on Thursday after China instituted new supportive measures, including restrictions on short selling and loosening of margin lending regulations. It had been experiencing a selloff for the last couple of weeks.

This made for the fifth straight down day for Apple stock. The shares were down four straight days in June. The last time Apple was down for five days in a row was between Dec. 29 and Jan. 5.

Read More Apple preparing record number of iPhones: Report

Sherri Scribner, senior tech analyst at Deutsche Bank, put out her own note predicting that iPhone 6 sales will slow through the second half of the year. In addition, she expects that the company will underperform in 2016.

"The success of the iPhone 6 has been driven largely in part by China," she told CNBC's "Power Lunch" on Thursday. "So if China is slowing, it's one of the bigger smartphone markets, it's probably going to be an issue, because that's where Apple has opportunity to gain some share."

Scribner is also concerned about Apple's ability to expand beyond what she estimates is a 40 percent growth rate this year. "That's going to be very difficult for them to see that type of growth off of a 40 percent base, considering how big they are," she said.

Disclosures: Neither Scribner nor her firm own shares of Apple.

—CNBC's Evelyn Cheng and Bertha Coombs contributed to this report.