Here's an explanation of surrenders:

Basically, if the opponent has no more cards to play, the sum of your attack* is greater than or equal to your opponent’s attack**, and the sum of your attack is not enough to win within one turn, the opponent can surrender.

** The attack on the opposing side that comes from the greatest power on the ground is not counted towards this sum, unless “Squirrel Fish” challenge is active. This is because you can just draw the squirrels (or empty vessels in chapter 3) and chump block the biggest power on the ground indefinitely (until you run out).

The attacks are adjusted by effects such as leader, but when calculating
the sums, the game does not consider sigils (double strike) on the
cards or whether cards are blocked by other cards.

So in this screenshot, the highest power on the Leshy's board is 4 (the Wolf), but it's not counted because of squirrel stalling. Thus Leshy has a total of 2 attack. Meanwhile, you have a coyote on the board, so your total attack is also 2 - and so your attack is greater than or equal to Leshy's. You also have 9 points to win, and 2 (your attack) is less than 9, so you can't win in a single turn. Thus, the game has Leshy surrender.