It's meant for humor, I think. Take a pathetic creature (one we eat, one renowned for cowardice - "chicken" is another word for coward!) and make them this powerful killer force! There's a lesson in it, too - don't needlessly abuse seemingly harmless animals.
I also think that it may have some basis in actual chicken behavior. I grew up around chickens. I currently live in an apartment and cannot have any, but one of my minor dreams is to be able to have a place in the country where I can raise my own chickens (for nice, fresh eggs, possibly for meat, too). My fiance's not too keen on the idea. Anyway, while hens are pretty passive and will run if you chase them, roosters can be quite aggressive. (Cockfighting exists for a reason). An agitated rooster standing up for his hens will scratch the crud outta you if you aren't careful.
I wonder if Miyamoto or Aounuma (dude whose name I cannot spell...) ever had any childhood experiences with aggressive roosters....