Under Armour is launching its new UAS sportswear brand this fall. Image source: SneakerPolitics.

Under Armour (UAA -1.72%) stock has handsomely rewarded patient investors over the past few years, despite a near-constant chorus of skeptics claiming shares of the athletic apparel specialist were overpriced along the way. But that's not to say Under Armour stock hasn't pulled back from time to time. In fact, I think even the most bullish, long-term investors would be wise to examine the reasons Under Armour stock could possibly fall.

To help you get started -- and as a longtime Under Armour shareholder myself -- here are three things that could do just that.

1. Additional debt/acquisitions

First, I'm sure I wasn't alone in raising my eyebrows when Under Armour not only spent $150 million to acquire app maker MapMyFitness in late 2013, but followed up by forking out another $560 million just over a year later to acquire fellow fitness app specialists Endomondo (at $85 million) and MyFitnessPal ($475 million). But because Under Armour was already investing heavily in both supporting its domestic growth and building out its global infrastructure, the company had to resort to using debt to fund those acquisitions. As of its most recent quarter, total debt had increased to $1.2 billion, up from $712 million at the same point a year earlier. 

To be fair, Under Armour revealed last quarter that, as a result -- and combined with the subsequent launches of its reworked Under Armour Record app -- it now boasts a world-leading connected fitness community of over 175 million registered users, with another 100,000 signing up every day. Also as a result, revenue stemming directly from its Connected Fitness platforms increased a solid 73% year over year last quarter, to $23 million.

However, that's still only a sliver of Under Armour's total sales. And though I know this incremental revenue stream will inevitably continue to grow, you can be sure the market would be wary if Under Armour were to drive debt up even further through additional acquisitions.

2. Impending gross margin headwinds

Next, one of the most exciting pieces of the buy thesis for Under Armour is its international potential. Last quarter, international revenue climbed 68% year over year, to $150.1 million, but still only represented 15% of total sales, leaving a massive runway for global growth. But expanding overseas takes significant investments to establish infrastructure, raw materials partnerships, distribution deals, and building Under Armour's own direct-to-consumer outlets -- all of which indicate Under Armour is poised to endure gross margin headwinds as it ramps its international expansion. 

Another headwind lies with footwear sales, which are also growing quickly for Under Armour -- up 58% year over year last quarter, to $242.7 million -- but tend to come with lower margins than Under Armour is accustomed to enjoying with its core athletic-apparel offerings.

That said, neither of these gross margin headwinds should be deal breakers for Under Armour investors, as the company is well aware, and can partially offset them through efficient cost management and controlling the product mix, to an extent. But over the near term, the market could certainly get spooked if it perceives an unhealthy drop in margins going forward.

3. Any stumbles in sportswear

Finally, this fall, Under Armour will officially launch UAS, or Under Armour Sportswear. This new brand will mark Under Armour's entrance into what CEO Kevin Plank estimates is a $15 billion market for sportswear, a category Under Armour has effectively eschewed to date. 

To help make the most of this style-centric opportunity, Under Armour hired fashion designer Tim Coppens as executive creative director of UAS. And though the initial reviews of the collection from industry watchers has been largely positive, it remains to be seen how consumers will react once UAS hits store shelves.

On one hand, if UAS can grab even a small slice of that $15 billion market from competitors, it could make an enormous impact on Under Armour's top and bottom lines, considering the company anticipates total revenue this year to be less than $5 billion. On the other hand, if UAS fails to gain traction -- and with the caveat that Under Armour can always go back to the drawing board if need be -- it could tarnish the brand, and cause investors to adopt a more skeptical view of Under Armour's ambitious plans to extend its reach.

In the end, I still love Under Armour, and plan to hold its stock for the foreseeable future. But I'll also be keeping a close eye on these potentially negative catalysts going forward.