Its because most boomers always say things like: "Oh you guys are always so easily offended and lazy and dont care about anything other than that phone!!!" so, the gen Z girl went, "Okay Boomer" and then all the boomers got super offended by it, which contradicts everything they said previously about 'not being easily offended' and such. Some boomers went to go as far to say, "Calling me a Boomer is the same as calling someone the N word." to which everyone immedetly dissagreed. But, I can say for sure that most poeple do know why they say it, and now its really more of a thing like, "If you say Im easily offended for no reason, if you say Im addicted to my phone, if you say Im lazy when Im not, then you deserve this 'OK Boomer'." I dont know what you were doing when you got called a Boomer, but I doubt it was any of those things. Maybe that kid was one of the few who actually didnt know what it meant, and Im going to apologise on behalf of that person.