Indeed this is an interesting phenomenon I experienced in new year celebrations in 2010, myself and a few hundred others were walking across the lower level of the suspension Bridge across the Douro in Porto, the one right next to town, and it started moving like this. At first we thought it was our imagination, we were maybe just drunker than we realised, though we hadn't had much to drink, my partner at the time and I, she was very religious and dropped to her knees and started praying, which was actually the best thing everybody could have done. Some others also had the brainwave to start running, this also served to counteract a little of the very wierd stegger-swing motion we were inducing on the bridge. I swear it was moving a good few feet from side to side.
When we got to the other end, we discovered there were police and security guards there, coordinating with police at the other end, regulating the number of folk on the bridge.
Standing where they were at either end, they could see the snaking wave movement of the bridge, literally waves of movement propagating along the bridge backwards and forwards along it's length like a snake.
Apparently this was well known to them, and something that always did, when large crowds were expected on the bridge, they just monitor the bridge for movement, and filter people off when it gets "Too much". As an Engineer I think that strategy has to include regular inspections of the bridge to ensure no signs of imminent structural failure, even minor, including for rivets dropping out from upper to lower levels, the size of a steel bridge rivet, dropping from that height onto a person is such it would be likely to kill, even through the roof of a vehicle when those are on the bridge (They weren't allowed that night). This was my main fear, I was worried rivets would start to fall when we were on the bridge.
Ive had some scary experiences in life, and that was one of the scariest ever :)