Apple's (NASDAQ:AAPL) stock is still positive in 2015, but the iPhone-maker has given back most of its gains. Year-to-date, it's up just over 4% -- but shares have fallen more than 8% in the last month.

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Should shareholders anticipate further weakness? Below are three scenarios that could weigh on Apple stock. Although a general market rally could always lift Apple's shares even if all three scenarios come to pass, these certainly wouldn't be seen as favorable developments.

The iPhone business could stagnate
Apple has enjoyed record demand for the iPhone in recent quarters. Last quarter, Apple's total iPhone sales rose 35% on an annual basis, while iPhone-related revenue surged 59%. Much of this demand is likely due to the iPhone 6 and iPhone 6 Plus -- as demand has risen, the iPhone's average selling price has increased. In the third quarter, the average iPhone sold for $660, up $99 from the same quarter last year. At the same time, Apple's management claims that the percentage of ex-Android converts is at an all-time high, suggesting that the iPhone 6's larger screen -- a feature once exclusive to Android handsets -- has enticed buyers.

But how long can the iPhone 6 upgrade cycle continue? During Apple's last earnings call, CEO Tim Cook pointed out that only 27% of current iPhone owners are using an iPhone 6 or iPhone 6 Plus, suggesting that there's much more room to grow. But that figure is somewhat misleading. In February, Consumer Intelligence Research Partners noted that while there were more than 10 million iPhone 4S sets still in use in the U.S., many of them were fairly new. Although it was originally released in 2011, Apple continued to sell the handset up through last September. At least in the U.S., 72% of current iPhone owners had purchased their handset within the last 17 months, suggesting that they may not be ready to upgrade.

And even if the iPhone business remains strong, growth could under-perform expectations. Apple sold an impressive 47.5 million iPhones last quarter, but that wasn't enough for Wall Street analysts, who had been looking for around 49 million.

China's economy could slow
Apple is increasingly reliant on the Chinese economy. Last quarter, Greater China generated nearly 27% of Apple's revenue, and almost 60% of its revenue growth.

The Chinese stock market has been tremendously unstable in recent months, which could have a negative affect on its economy. China's Shanghai Composite has fallen nearly 30% since the middle of June. Chinese regulators have responded to the turmoil by suspending the trading of many stocks. Some have resumed trading, but many are still halted. The sell-off has slowed, but the long-term effects are difficult to gauge. Given its exposure, economic turmoil in China could take a toll on Apple's sales, and by extension, its shares.

Other segments could remain tepid
The iPhone generates nearly two-thirds of Apple's revenue, but it isn't the company's only product. The Mac business has been a consistent performer, gaining share even as the PC market contracts -- but the iPad's performance has been disappointing, and the Apple Watch's reception has been fairly modest.

iPad sales have contracted for six consecutive quarters, and it's now the company's second-smallest segment (behind iPhone, Mac, and Services). Still, management remains optimistic, believing that a push into the enterprise space and enhanced features will help it return to growth. But Apple's management has been downplaying the iPad's weakness for years -- initially blaming it on shifts in channel inventory. If iPad sales remain muted or continue to fall, Apple's dependence on the iPhone will only intensify, increasing its risk profile.

The same is true for Apple Watch. It can't be called a flop, as Apple hasn't released exact unit sales, but the conversation around the product has turned negative in recent weeks. Late last month, Bernstein Research's Mark Li called Apple Watch sales disappointing. Other analysts, including Piper Jaffray's Gene Munster, have lowered their estimates for Apple Watch sales. Apple Watch isn't material to Apple's business -- at least not yet -- but it does represent Apple's growth prospects. If it under-performs expectations, Apple's valuation (and its stock) could slide.

Sam Mattera has no position in any stocks mentioned. The Motley Fool recommends Apple. The Motley Fool owns shares of Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not 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.