One of my rabbits has suddenly started bullying and become agressive to his female bonded partner!?

I originally had my little girl bunny for two years and then rehomed a 'fixed' boy bunny for companionship. They bonded instantly and up till recently hardly ever sat apart and continuously licked and groomed eachother. They've been bonded for almost a year now, however recently my boy bunny has started growling and not letting my girl rabbit get to food. They've been sitting apart for the past two weeks and I'm worried that she isn't getting enough food. I try and sneak her treats but he always runs over and chases her away to get the food. He never did that before! They both went to the vets recently and all was fine, although they were still 'in love' then. Has anyone else experienced this? Both bunnies have been fixed for years, have a large run and hutch and are never shut away and contained in a small area. :( :(