Unlike dying, which connotes a degree of violence and urgency, passing away is a situation where you slowly leave the world. It can be precipitated by lying on the floor beneath a cubicle desk. Passing away is a way to escape the madness that may occur around you.
-"What are you doing this weekend?"
-"Oh I'm passing away."
"This madness. I need to pass away."
-"Where are Jackie and Jen?"
-"They passed away at -100 o' clock."