My theory is that it is the increased brain work from navigating the store, comparison shopping, keeping track of what you need and what you’re spending, plus walking.
With my worst store tank happening during Back to School shopping (which is essentially the worst riddle+Where’s Waldo+roller derby against other frantic moms who need Blue Folders with NO TABS bc the teachers are that specific so everyone in the county needs the same flippin’ color folder and there are never enough), I think the increased brain load theory makes sense. I think I’ve read that your brain uses 60% of your carbs/sugar/whatever, but I’m sure someone will clarify that if I’m wrong. 