Friday was a particularly brutal day in the market, with the Dow Jones Industrial Average down more than 200 points at the low. Investors perusing the headlines and looking for a cause for the drop would find articles pegging soft earnings from heavyweights Microsoft (NASDAQ:MSFT), General Electric, and McDonald's as the culprits.

The software giant's 22% drop in net income led the tech sector lower, with the tech-heavy Nasdaq Composite hitting its lowest level since August. Of course, Apple (NASDAQ:AAPL) is the largest single component of the Nasdaq, comprising more than 12.5% of the index. Shares of the iPhone maker closed just under $610 near its lows, down almost $23 on the day, on no news specific to itself. That was a decline of 3.6%, for those of you keeping score at home, underperforming its software rival whose results helped spark the sell-off.

AAPL Chart

AAPL data by YCharts

In essence, Microsoft's weak quarter triggered pessimism in tech, causing Apple to drop like a rock. The stock is now well into correction territory, down 13.5% from its all-time high set almost exactly one month ago. Here's why Apple's sell-off on Friday was absurd.

The whole reason Microsoft's quarter was so bad was that the PC market is simply a stinker right now. Global PC unit shipments fell 8.3% in the third quarter. Weak demand for PCs is hurting a slew of companies in the value chain. Even chip giant Intel (NASDAQ:INTC) just put up uninspiring figures because of the languishing PC market. The growth in its data center division wasn't enough to offset declines in the PC client segment.

The thing is that the whole reason the PC market is so lackluster is that consumers are shifting those spending dollars to mobile devices. We're talking about smartphones like the iPhone and tablets like the iPad cutting into those PC unit figures.

While the iPhone doesn't have the top spot in market share -- that title goes to Google (NASDAQ:GOOGL) Android -- the iPhone is largely credited with sparking the current wave of smartphone adoption by popularizing the now-ubiquitous capacitive touchscreen. The iPad does have the top spot in the tablet market and is unambiguously responsible for igniting tablet adoption.

These are the primary reasons consumers aren't buying as many PCs as they used to, and it's no coincidence Apple refers to these as "Post-PC" devices. That's why it's so nonsensical for investors to dump their Apple shares on Microsoft's woes.

Evan Niu, CFA, owns shares of Apple. The Motley Fool owns shares of Apple, General Electric, Google, Intel, McDonald's, and Microsoft. Motley Fool newsletter services recommend Apple, Google, Intel, McDonald's, and Microsoft. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.