My son is 5 1/2 months and does it too. He actually does arms and legs sometimes. When I put him in his bed at night, he can be asleep and as soon as he's ruffled just a little by putting him down, he slams his legs down. I think it's just something that they do, although the person who called it a disorder has me a bit concerned now and I think I'll confirm that it's normal for that to happen.