Probably wasn't side to side. There have been reports in the past of aerobics on one floor causing vertical annoying vibrations on another floor, sometimes several stories away.
If one measured the vibration modes in this case, he'd find some that had column axial deformation that allows the energy to transmit vertically. In the mode that caused the problems, there probably was significant floor bending at the activity and at the floor with complaints. This structure just happened to have at least one mode like that, and its frequency was an integer multiple of the jumping frequency. Very cool.
Resonance is incredibly powerful phenomenon. A few years ago, I was at the cantilever (120 ft IIRC) tip of a very wide balcony (hundreds of ft--forgot how many, but 600 ft is ringing a bell) and bobbed up and down at the natural frequency. My pal who was at least 200 ft away said the vibrations were strongly perceptible. That's a 200 lb guy vs a several million pound cantilever! It was lightly damped with no audience and I was able to hit the natural frequency directly, not with a higher harmonic.
As an aside, that's why the AISC Design Guide 11 says to never, ever ever have a structure with a floor natural frequency below 3 Hz. You don't want people seeing how much they can make your structure vibrate! It's fun on a structure below 3 Hz LOL.
Apparently Nikola Tesla was experimenting and attached an oscillating device to a building he was doing experiments in. He tuned it to the building frequency and according to reports, almost brought the building down.
