To pile up is to grow into a heap or a backlog. Let the work pile up.
It is used of snow, of washing up and of anything nobody is dealing with. Things pile up quietly, which is why the size is always a surprise.
1phrasepile upTo grow into a heap or a backlog.
“The letters pile up whenever anybody goes away.”
pile up is an action.
Practised in Phrasal Verbs