With other numbers, non-date numbers, we put the numbers representing the most quantity to the left, and numbers representing the last quantity to the right, eg 1 hundred, ten and 1 would be 111, where the number representing 100 qty comes first from the left, and each position moving to the right, represents a smaller and smaller amount.
Since years are longer than months, which are longer than days, the YYYY-MM-DD format actually follows the same convention that we commonly use for all other numbering systems, big on the left, small on the right.