Why does my male cat viciously attack my older female cat only when he eats dry cat food?

He is overweight and would eat 24 hrs a day if I let him so I have to monitor his intake carefully. The older cat isnt anywhere near him when he seeks her out and attacks her after eating dry food. I switched him to wet food and he stopped doing it but lastnight he got into the dry food and immediately attacked her after I took it away from him.