Apple's stock cannot stop tumbling, hitting a new worrisome benchmark this week. The company's share price dipped below $400, its lowest level in 16 months. It also represents a steep drop — 42 percent — from an all-time high of $705.07 in the heady days of September 2012.

The curious part is that Apple's much-anticipated earnings report for the first quarter isn't even out yet. So why are investors so jittery?

A few ideas are bumping around. First, Cirrus Logic, which manufactures audio chips for iOS devices, announced a revenue forecast with numbers well below Wall Street expectations, fueling concern that Apple's iPhone and iPad sales have dropped.

Cirrus, for its part, says one of its customers — it won’t say which — has moved on to a different audio component, which could mean that Apple is merely dropping Cirrus as a supplier. But as Nigam Arora at Forbes notes, "The stock market so far seems oblivious to this alternate explanation."

Even if Cirrus' results do reflect a drop in iPhone sales, they "shouldn't create such a hard hit," says Romain Dillet at Tech Crunch. Another factor may be that Apple hasn’t announced a new product since the iPad Mini in October 2012. Product announcements tend to bump stock prices, and such a long gap between surges may have left investors feeling antsy.

Competition could be another factor, with two much-hyped products — the HTC One and Samsung’s Galaxy S4 — hitting stores soon. 

But beyond these short-term factors, many are tying today's dive to a broader slump in confidence in Apple's prowess. Fears that Apple has run out of creative juice to produce another game-changing device has only deepened, and the pressure on CEO Tim Cook to pull another iPhone or iPad out of his hat is increasing.

"The CEO is not a product visionary," say Jay Yarow and Henry Blodget at Business Insider"Apple has cash coming out of its ears, but no clue what to do with it."  

We'll see what happens when Apple releases its first-quarter report next week. For now, every week brings more evidence that Apple is no longer the golden boy on Wall Street. Indeed, on Wednesday the company lost its title as the world’s most valuable publicly traded company to Exxon Mobil.