Consistent calm

I have a very calm seven-month old golden retriever. He's not very reactive at all and he's not very outgoing. He's not interested in greeting everyone on our walks and we can successfully walk by every house that has a barking dog in the backyard without incident. However, once in a while he does react. He'll growl at someone he sees, he'll randomly start barking and growling at a stranger, he'll raise his hackles when a dog walks by. It's maybe 1 in 30 encounters, or maybe once a week that it happens.

Because his triggers don't seem obvious to me, does anyone have suggestions for this? I don't want this to be 1 in 30 and then 1 in 20 and then 1 in 10... It just seems so random. Once it starts, the only way to stop is to just keep him moving (which means I'm sometimes pulling his harness) until we're away from whatever it is that is triggering him.