Fallout conspiracy theory: Most of the skeletons in funny poses were placed like that by bored wastelanders long after the war.
I'd like to think it's a collaborative process. One wanderer puts the skeleton on an old beach chair. Later, another wanderer sees it, chuckles, and gives the skeleton a sunhat. Another one gives it a martini glass with an umbrella. And so on.
Tbh could 100% see this developing as a wasteland folk tradition to respect the dead who their loved ones never got to bury. Long journies that never came home, so we comfort the nameless dead by sitting them upright to be remembered. Decorated, appreciated, spoken to by lonely travelers while they donate a scarf, or a vest, or a threadbare blanket to the unknown soul.
Can also see whatever region it started in having a whole philosphy about remembering those before to learn from their mistakes, and how laughter is the best tool to open a mind to learning. If we can laugh at the dead, we can understand how rediculous it is to Cease. Suddenly or slowly, we laugh cause We Felt That. And now the bones are no longer revolting, they can be studied. Basically acitvely squashing the Corpse Taboo a lot of human cultures develop as a foundational principal of said culture.
And what one group does is observed by others. Sometimes mimicked, sometimes even taught to visitors... but in time, observers get it wrong, different regions didn't get to ask those who started the practice and had to ask themselves; why do we set up dry bones like they're still people? And answer it to their own satisfaction.
... and if they're the Assumptive and Loudmouth sort, start proclaiming their answer to others at every oppertunity. Irreguardless of correction.
And that is how culture drifts from place to place, different localities having different views on the practice and their own particularities on what's okay or not okay to do with bones.
