1) don't let people crowd around you
2) hold onto your camera by the lens
3) backpacks put valuables in your blind spot
The modus operandi here of the street thieves is a well documented practice: present cardboard/papers up to a person's chest and let your hands go to work below the cardboard. Pockets for wallets used to be the common target. Lenses from DSLRs make a new, easy target.
I've seen gypsies in places like Florence try to use this exact tactic where there is a mother + children roaming for prey.
But really the problem here is (1) - he let himself get surrounded and was thus in a situation he could not hope to control or survey. I can't tell if he knew he was a target but if he did know then he should really have thought more about what was valuable to him.
When walking around cities where I have reason to be careful, my camera and camera bag are cross strapped, one for each hip and a hand is always on each. The only time I use a backpack for camera gear is hiking.