126 Hmm a little more information might be helpful. Lets follow the triangle and see…
Knowledge: Has the dog bitten any other dogs? Was it just that one time? Are there any other situations when this happens? Do you ever see any body language when hes with the other dogs that might indicate fearful/defensive behaviors or otherwise? More details might help piece together a scenario and a trigger.Health: Any health issues we should know about? Ear infections, tooth infections, etc?
Attitude: Are respect, patience, calm confidence and poise being displayed during training, handling, etc
Golden rules: Timing, motivation, consistency. Have you watched him as he plays with other dogs. Timing is especially important here. If you can catch him in the act you might be able to intervene immediately. With the proper motivation and consistency you might be able to correct this behavior. Assuming hes giving you a sign before he bites.
Pack structure: I noticed this bite took place in your home and after you had left. Are pack structure rules being followed in the house by all members of the household? Toys, beds, food, affection, going out? Are the members of the household in control of all those things and are they initiating walks and play sessions? If he is being assertive when initiating a play session with another dog it could be directly related to pack structure.
Restless spirit: Are dog group sessions his only workout for the day? What his exercise routine like?
Anxiety: Any anxiety issues. Chewing, destructiveness, peeing/pooping? The idea here is to remove as many stressors in the dogs life as possible.
Housebreaking: Accidents in the house?
Obedience: Obedience could help alot in this situation. If you see him getting into a situation you dont like you could recall him, let him reset and then let him back into the group. How far along in the obedience are you?
As you can see a little more details might help us put together a picture of whats triggering him. Troubleshooting everything in the triangle along with a better picture will help us better see whats happening here.