pile-up

Short definition from Wiktionary

Noun

1. 1
   A pile, a group of people or things which have piled up on one another, especially: A pile of crashed vehicles due to a traffic accident.
2. 2
   A pile, a group of people or things which have piled up on one another, especially: A pile of tackling players.
3. 3
   An accumulation that occurs over time, especially one which is not welcome.

Verb

1. 1
   To form a pile, stack, or heap.
2. 2
   To collect or accumulate, as a backlog.