grace does that too. she's more leary of men than women. she doesn't discriminate a whole lot. she'll growl at women too. she's just more intense around men. i've heard female dogs think they're people are their puppies and it's a motherly instinct they have. that and being a rott, she's got that protection instinct too.
i'm wondering if your teacher at class can show you how to posture yourself differently to make Bryce think it's ok to not be so protective.
The way the Dog Whisperer talks, it's just as much out behavior as it is the dog's. I notice that the more autoritative I stand or the more relaxed I feel, the less aggressive Grace is. Not that you weren't relaxed, but maybe he felt since you were looking at something else, he needed to take over watch for a bit. just an idea.