I hate to write this, but I had a black lab that was very very well trained and showed zero aggression. As a pup I used to put his food in my hand and put it in his mouth, also used to bang pots and pans and shout as he ate. (relate happy times to loud noises, makes them less gun shy)
When he was still young around 5 he took a swipe at me out of the blue. Way out of his character. Took him to the vet, he was very sick, very sick.
Take the advice others mentioned above and have him looked at.
If he is healthy, maybe he forgot the rules and needs a reminder like sinker said.