Incomprehensible Re-Arrangings

Why do dogs dig? TB knows perfectly well that the 3 rats are no longer in His compost heap because it saw Him throw them away. But it spent hours yesterday re-arranging said compost heap in a futile effort to find them. Why? True, TB was happily employed and emerged extremely muddy and gloriously, riotously pleased with itself but without a rat.

On the other hand He was horribly, furiously annoyed. (Now that Myself can understand. But not the digging.)