Lots & lots of sensory input. Fluorescent lights in most cases which some people are allergic to (AS or NT both.) Messes. Screaming kids. Rude people (taking up the whole aisle, or pushing carts into others, cutting in front of line, having 50 items in the 10 item or less line, checks and credit cards in the cash only lane...etc.)
Then for people who dislike too many choices, in the Western world supermarkets have too many choices of things to buy. Specials, complicating decisions - buy the one on sale not knowing if you'll hate or be allergic to it, or, stick with the tried and true brand, and pay more? So, it's also a bit unpredictable.
Germs, if you're a germophobe (as I somewhat am) - people digging into the barrels with their hands and eating, licking their fingers, going back for more - people opening packages, digging through with their hands, eating and closing it up as if nothing happened, leaving it for someone else to buy - cashiers who lick their own fingers before giving you your change or a bag.
And that's after you've battled the parking lot or taken a cab or bus to get there to begin with!
Which reminds me...I need to do the online shopping Lol.