This does not surprise me. A couple of things come to mind when I think what precipitated the slump.
I have kids in their 20's and teens. I would think that is a large segment of Under Armour's market. Those age groups are known to be very fickle with their clothing choices. Remember, Hollister or Abercrombie & Fitch? Those brands are "so yesterday" with my kids.
Second, their lines are priced on the high side of things for kids of a younger age.
Third, what is their competitive advantage? I have seen a lot of knockoffs of their clothing.
When you serve a younger and fickle market, you can be a hero one day and garbage the next. Thus, it does not surprise me what has happened to their shares.