Ok, so I just watched the video again, and yeah, you're all right that it was a monkey see/monkey do affect, but with the volunteers, not with the girls. That guy in the black jacket on the right side was just itching to grab people for whatever reason, and once he started, the two guys in white on the left just started following. After a minute of that, it seemed everyone figured that's what they were supposed to do.
The ladies, as has been pointed out, were basically just staying where they were put for a minute or so until they caught their breath.
It'd be interesting to know where these volunteers came from. I'd assume these weren't completely green volunteers with no track/xc experience, especially the ones dressed alike in the blue shirts. I'd also assume that most/all of them worked the finish for the previous three races, and didn't engage in any such antics. I think the guy in black was probably new, and the others, despite possibly some experience, just followed him.