B.C.
It's probably for the same reason they burst into tears when they first start visiting Santa Claus at the mall - they suddenly realize the adult talking to them isn't Mom or Dad, they didn't expect it and the surprise caught up with them and overwhelmed them a few minutes later.
I've seen this happen at the grocery store too.
A toddler goes to grab Mom's leg while waiting in the checkout line and got the wrong person.
Suddenly the kid looks up and suddenly realizes it's NOT MOM!
Waahhh!
At Costco we always kept our son sitting in the cart just to keep him safe.
Not because he would wander but because some people have really filled up carts, and they'd have a hard time seeing a small kid.
Small kid not paying attention + shoppers not paying attention (many are on phones) = run over toes or bumped kid.