Useless knowledge

"How come they don't attack each other? How do they manage something as complex as social cohesion with their bodies and brains literally decaying beyond the point of corporeal cohesion?"
"Actually, they do. And it took us quite a while to figure out on what grounds they form their friend/foe views of the world. You will never guess what it turned out to be."
"Ethnicity?"
"Not even close."
"Nationality?"
"Nope."
"Something relating to geography?"
"Still nope."
"Ideology?"
"You're in the cold."
"Religion?"
"Getting warmer."
"Preferred social media platform?"
"Heh. Extra bonus points for lateral thinking, but no, not that either."
"Okay, I give up. What makes the zombies identify each other as either friend or foe?"
"Turns out it had something to do with which nineties boy band they listened to before the end of death. The exact nature of how they divide themselves into distinct social groupings is unclear, but if you know their taste in this particular genre, you pretty much know who they'll regard as friends. And if you are versed in the specifics of the videos to the songs, you have a direct line of understanding into the subtle infightings of the various zombie factions."
"You're kidding me?"
"Nope. They tried to pin it to just about every other explanation the universe had to offer, but this was the one model that actually worked."
"But... boy bands from the nineties? And the music videos of that era? I don't understand?"
"It just goes to show that there is no such thing as useless knowledge. Now let's get moving. I'm afraid these full graves are quite likely to empty themselves in about an hour or so, and we don't want to hear this particular rehearsal again."
"Rehearsal? ...right. I'm already gone."