Eddie shows a flash of aggression

I wish I was a behaviorist and could look into Eddie's wee brain and figure out what's going on. Last night at the little-dog meetup, a nice shih-tzu came up to give him a sniff, and he wheeled around and bit him right on the face, leaving a red mark dangerously close to his eye.

I was stunned. Eddie has toy-hoarding issues that I've worked hard on and successfully controlled for years, but there was no toy involved here. The other dog may have been a little pushy, but Eddie overreacted by biting.

I put him in an X-pen for a while to let him cool down, then brought him out on leash to see if he'd improved. He aggressively tried to get to two other dogs.

My first thought was an injury, but I could find nothing on his body and he gave no indication of being in any pain when I touched all over his body.

I can't figure it out. I've lost count of the number of obedience and agility classes he's completed. I've been taking him to big-dog and little-dog meetups for years. He's not the most playful pooch in the pack, but he's never been aggressive when no toys are involved.

Is it his diet? I recently switched to Canidae grain-free all-life stages, which ups the protein content. But that shouldn't cause aggression. Is it a lack of exercise? We had a severe cold spell for two weeks and he didn't get long regular walks. But he wasn't exactly bed-ridden. We played a lot around the house, at my office and in agility class.

I'm doing something wrong, but I just can't figure out what it is. I've tried as hard as I could to make him a well-rounded dog, but something has gone awry.

I wish I knew.