What the traffic did, accidentally? (5,2)

I believe the answer is:

piled up

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'did accidentally?' is the definition.
Although both the answer and definition are past participle verbs, I can't understand how they can define each other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for piled up that I've seen before include "accumulated" , "Heaped" .)

I've seen this clue in the Evening Standard.

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