Corolinth wrote:
The explanation is simple enough for a first-semester physics student. The box was rolling down the ramp with a speed equal to that at which the conveyor belt moved up the ramp.
For it to happen randomly because the box hit a bump, kink, or other deformity is pretty unusual (though certainly not impossible or even implausible), but a far more likely explanation is that a bored employee just started playing with the box until he got it to roll just right.
I'm too tired right now to think through this entirely (damn you Skyrim. I didn't need to be doing quests until 330 AM) BUT, if the center of mass is slightly variable, and the mass is more concentrated in the center of the package, I think this more likely. I think the tolerances of when this will occur are greater under those conditions than if the center of mass is off center, or evenly distributed through the package.