it's because when you get up for a walk, or wash the dishes, you use different parts of your brain. it's the same as knitting (i.e for an experienced knitter, not a beginner) while watching TV. talking to another person, especially about work related issues, pushes the previous stuff onto a very short stack, and the stack overflows immediately. often times it's not about trying to get the stuff back in mind as it was before, but having to go through the entire series of steps again to get there.