Under Armour is susceptible to fluctuations in consumer demand following turns in the economy. UA's sales may slide as the global recession continues to hurt retailers. Additionally, Maverick Capital, which owned over 7% of Under Armour, unloaded its entire position in November 2008 because of negative expectations.
Increased Competition. As Under Armour ventures into new product lines, the sleeping beasts of Nike and Adidas (and others) are not going to give up market share easily. With its competitors being much larger (Nike is 10x the size), Under Armour may have an uphill battle to gain market share in its new apparel and footwear markets. UA's entry into highly-competitive turf will undoubtedly energize these competitors and make further market share gains for Under Armour difficult.
Valuation. Reaching $54/share on 7/5/07, Under Armour is currently trading at $54 or 67 times its earnings. Even with high compounded growth, the high valuation makes the stock price susceptible to even minor mishaps along the way.
Susceptibility to consumer trends. Should the retail spending at Dick's Sporting Goods or Sports Authority wane, Under Armour, which attributes 40% of its revenues from these two sources, may suffer.