Pile Up Meaning

(idiomatic, intransitive) To collect or accumulate, as a backlog.

Example:   The kids piled up their boots and coats by the back door.‎
1898, J. Meade Falkner, Moonfleet, Chapter 4
  And still the hours passed, and at last I knew by the glimmer of light in the tomb above that the sun had risen again, and a maddening thirst had hold of me. And then I thought of all the barrels piled up in the vault and of the liquor that they held; and stuck not because 'twas spirit, for I would scarce have paused to sate that thirst even with molten lead.
1977, Agatha Christie, An Autobiography, Part II, chapter2:
  If I close my eyes I can see Marie today as I saw her then. Round, rosy face, snub nose, dark hair piled up in a chignon.
  The requests piled up while she was away.‎