In going through the news reports, I can take a guess about what happened...
Probably the valet was in a super big hurry, trying to stay on top of a rush of customers. He parked a Mercedes and shifted into park so quickly that it didn't catch. He got out, saw the car begin to roll forward, then did all he could to avert disaster. He ran to stop it. In a panic he stretched his leg too far and hit the gas instead of the brake. In the process, he made things a lot worse. The car forcefully rammed into a crowd of people who were hanging out alongside another parked car. The victim who died was pinned between 2 cars.
I know how a car can roll away like that, because I have experienced it myself. It was the same sort of thing. We were going down in flames. People were waiting too long to get their cars back. I was moving and running as fast as I could. I shifted a car so quickly into park that it didn't actually shift into park. I left the car and then saw it rolling. It was about to t-bone another parked car.
I saved it!
It was a close call though.
It taught me that I always had to take an extra two seconds to verify that a car had actually been immobilized. Even if we were completely "in the weeds," I had to take that extra 2 seconds to be certain the car was securely in its parking place. This, for me, was an absolutely mandatory step, no matter how busy I was.
This is something that valet operators should be teaching their employees.
Valet workers need to take that extra 2 seconds to be sure a car isn't going to roll after they get out of it.
And people need to be aware that valet parking "ramp areas" or valet staging areas are dangerous places. You cannot let your guard down. And standing in any space where you could be pinned isn't a safe practice.
Getting pinned against another car in a valet parking staging area is something that can and does happen.
When I was valet worker, I trusted none of my co-workers when it came to their ability to drive and handle cars perfectly every single time. No way did I trust them. I never stood behind or in a front of a car where there was even the most remote possibility of getting pinned. Nobody should trust them.
It didn't matter if they were brand new rookies or the best and most experienced among us. There was no way I was going to risk getting my legs crushed.
Nor did I trust any customer pulling into or our of our staging area.
We had old people who would suddenly hit the accelerator too hard and jump the curb.
I had a customer who very nearly ran my foot over as he aggressively drove away.
Often in valet staging areas there are valet workers with almost no valet experience and even minimal actual driving experience. They are prone to be nervous and unfamiliar with the cars they are driving.
Bottom line: It's simply a dangerous place to be. You cannot trust that you will be safe in a valet parking staging area. Stay aware. Take steps to minimize your risk of getting hurt.
This is an awful tragedy. I feel bad for all involved here. And that valet is probably going to need a psychologist to deal with what he has done.