I had the same thing happen a couple weeks ago from my RIR mighty rooster. I've had him since he was just a chick, and he has been great until recently. I let all my chickens free range for at least a few hours every day, and it was time to put them in their run, and I took feed and they were all following me to their home and for some reason he ran up and just bit me on the back of the leg. Brought the blood even through jeans. I kinda wrote it off and maybe he seen a bug or something (ladybugs are bad in my area right now) and didn't make a big deal. But it seems like ever since that incident he is just cocky as ever and acting like he is gonna attach every time I'm around him. Just today he even attacked my cat which he has been raised with and never bothered before, and he has fluffed up at me a few times and once he fluffed up and came toward me and I happen to have a plastic leaf rake and just pinned him down with it and held him for about a minute or so. That seems to have helped some. But if he does it again he is gonna be in with some dumplings or something soon. He is just now around a year old.