On stepup, shards will launch a task to drop temporary aggregation collections left over from previous terms. This asynchronous task is launched while still in drain mode, and will perform local writes. These writes will fail with NotWritablePrimary error if they race in a way that causes them to execute before the node has become effectively writable primary. This wil lcause temporary collections to be left over, at least until the next stepup (if the race does not happen again).