No expert, just my own experience with Titan, but sounds like something that Titan used to do when he was younger. He didn't growl just barked. I woudl assume that if it just started out of nowhere then it is more likely his maturing and trying to figure out his role on walks when random people approach, which should be your job not his.
What do you do when he does this? Do you ignore it, coddle, anything?
Commander Charly Titan (GSD)-15 Jan 2010
Lady Athena (BB)-2011-2012 (RIP baby girl)