I think it's just a fluke. The guineas and chickens are housed together and some roosters prefer the guinea hens. I don't think you can make it happen or count on it to happen.
There was a thread a while ago about a guinea/chicken (guin) named "Miracle". I dont remember the exact name of the thread though. If you find this thread you might be able to get some information, i have also heard that it is easiest to cross an araucana roo with a guinea hen.