worms in my chicken! how?

I bought a 10lb bag of chicken legs and thighs at the store. i separated it and put about 6 pieces in freezer bags and put them in the freezer until ready to use. they've been in there about 3 weeks. so i took one of the bags out to thaw out. when thawed, i put the pieces in a glass baking dish and put them in the oven at 350 degrees for about a hour. when i took them out, one of the chicken thighs had about 4, 2 inch what looked like round worms trying to crawl out of it! they were dead, and i didn't see that in any of the other pieces. how did round worms get in the chicken thigh? i threw it all out of course. i guess when the meat started to get hot in the oven, they tryed to crawl out. has anyone ever heard of this happening? i know if meat gets left out, flys can lay eggs and maggots can get in there, but, for one thing, this chicken was frozen for 3 weeks. so, wouldn't you think they'd be dead? they were about 2 inches long. sound's crazy.....