In one word - China. Most of the growth in Apple revenue the last two years has come out of China and if the Chinese economy slows markedly, it only stands to reason Apple sales growth there will suffer. Less iPhones sold equals less profit.
Should an investor worry?
There is a real concern China's woes can impact Apple's sales. However, at current pricing (near 110) - the 'troubles' are accounted for and you can safely invest in Apple. The huge money chest the company has on the balance sheet will shield you and the dividend will only increase in future quarters.
Buy Apple before earnings.
Short term rating = BUY
Long term rating = STRONG BUY