Another Monday, another Apple (NASDAQ:AAPL) sell-off. Starting the week with losses of over 1% on a flat market day is simply standard procedure for Apple shares these days, and today is no different. The iPhone maker is facing two competitive threats this week, and investors are beginning to think that rivals are starting to catch up.

Wireless carrier AT&T (NYSE:T) has now announced official pricing and availability of BlackBerry's (NASDAQ:BBRY) new Z10 flagship smartphone. The first BlackBerry 10 device will launch on March 22 with standard high-end pricing of $200 on contract, which is the same starting price point of the iPhone 5 and other flagship devices. Ma Bell will begin taking pre-sale orders starting tomorrow.

Keep in mind that AT&T has always been predominantly an iPhone carrier ever since it enjoyed iPhone exclusivity for the first few years. Since then, the iPhone's percentage of total smartphone activations on the network has steadily risen and reached an all-time high of 84% last quarter.

T Phones Labels

Source: SEC filings. Calendar quarters shown.

AT&T has been trying to wean its reliance on the iPhone since it carries such pricey subsidies. However, the Z10 similarly carries a hefty $600 retail price, which implies that AT&T is still on the hook for an expensive subsidy on Z10 sales. There's very little room for non-iPhone devices, as other OEMs divvied up just 16% of smartphone activations last quarter.

More important than the Z10 is Samsung's Galaxy S IV that is set to be unveiled on Thursday at its "Unpacked" event in New York. The South Korean company's Galaxy S Series is the most viable threat to the iPhone, even though Apple is still able to easily outsell Samsung's flagship on a worldwide basis. The next-generation model will likely give Samsung a boost in the short-term, though, ahead of a possible iPhone launch this summer.

With two rival flagship smartphones either being launched or unveiled this week, investors are letting fear get to them by selling shares and ignoring the fundamentals.

Fool contributor Evan Niu, CFA, owns shares of Apple. 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.