What about clothing? Similar to sleeves if any are used in training? Same color? How are arms being held? Bent at 90 degrees because they are holding a glass?
Additionally...people tend to think all barking is aggressive. NOPE! They are communicating. I hold "conversations" with my dogs while they talk back with different barks. I can tell when Sabo is being a stubborn butt head with his high pitched YUP! bark that he makes. I can tell Kia's "Play with me now bark!" at Sabo from around the corner.
Dogs barks communicate ALOT. If you are into Sch then you also know that teasing with a rag to elicit a bark and then subsequently rewarding said bark with the rag to tug on is common. Could this have just started in training? Could your dog think that barking at the men may mean that he wants them to start to play with him?