Supermarket Sweeps

What's with people pulling their shopping carts in the supermarket, instead of pushing them? And they do this while they have a family conference or coffee klatch in the middle of the cereal aisle, blocking everyone from getting around. Do they think by standing in front of the cart, pulling it, they have better control of it? Or do they think that it gives them the right of way over others? Maybe they think that there's some kind of law of physics involved which makes the cart move faster when pulling rather than pushing. It could be nothing more than somebody observing somone else pull and the herd mentality went into effect. I notice young people and middle aged people doing more cart pulling, as opposed to kids and old people. When did this start?