Here’s a question for people who believe that the stock market is rational: How do you explain what’s happening to Apple? The market value for the iconic electronics company has plummeted about $150B, or 23%, in less than two months as its stock price dropped from a high of more than $705 a share (which it touched on September 21) to yesterday’s closing price of $542.90. There’s wide agreement that this has virtually nothing to do with Apple. You can’t blame such a big drop on the widely reported snafus with Apple’s Maps application, or disappointment that the iPad Mini costs too much and doesn’t have a Retina screen. The stock is dropping because investors had unrealistic expectations. They bet that Apple would continue to hit grand slams, and are belatedly realizing that they may have to settle for a stretch of mere home runs and triples.

This shouldn’t have come as a surprise; the company’s grappling with issues that anyone could have anticipated for more than a year. People who attended January’s CES consumer electronics trade show in Las Vegas saw abundant evidence that competitors, especially those allied with Google, were developing attractive alternatives to the iPhone and iPad. Even though Apple popularized smartphones and tablets, products that continue to generate growing profits, it doesn’t have a monopoly on the concepts. And it shouldn’t be a surprise that Apple has run into so much trouble with the next product that was supposed to re-define consumer electronics: an easy-to-use television set that blends video from conventional television as well as the Internet. Cable and satellite companies, and Hollywood, continue to control TV, which is the main source of their profits. They have no incentive to let CEO Tim Cook stand between them and their customers, and he hasn’t shown that Apple has enough muscle to bust the status quo.

For all of Apple’s undeniable strengths, if you believe that the company’s now undervalued — that its falling stock price represents a buying opportunity — I’d like to know: Is there a reason to, as Apple would put it (but English teachers wouldn’t), think different?