Increasing accumulation. A pile up always need to be handled or maneuvered around. Imagine a heaping pile of whatever is being discussed : pile of laundry, school work unfinished, work chores to do, dirty dishes in the kitchen, emails to respond to etc. It refers to a conglomeration that slowly grows into an unweildy backlog.

“There was a car accident on the highway and cars just piled up behind it.”