What makes a dog become food aggressive?

We have two dogs, one male one female. They got along fine and then the male became aggressive. The male was not socialized except with my female and so he doesn't do well with new people or dogs. I noticed one day that he would be aggressive whenever the female would even walk by the food bowl. His owners leave food out 24/7 and he has three bowls that are always filled. Two in the kitchen and one in his bedroom. The female doesn't eat his food because she knows I don't allow it (it causes vomiting and diarrhea). Then when I feed the female the male will come up and start eating out of it too and then get aggressive when she continues to eat from her own bowl. He also eats his food like he hasn't eaten in days. I can't do anything about his aggression because it's not MY dog, I wouldn't get consistent cooperation from everyone in the house to work with him and I don't have money for a professional. I just want to know why he became aggressive and acts like he never gets fed. Although the male is the alpha (I think) my female is never afraid to challenge him. I think she was alpha first but then the male took over because of his aggression. So what do you think? I'm not looking for the 100% correct answer to my situation just opinions and a general idea about why the male is aggressive.