Bouncing happens more often on fixed gear bikes. The cranks and pedals are always turning whenever the wheel turns (there's no coasting). So, at a faster cadence, if the rider's timing is off, the pedal hits the bottom of the revolution and starts coming back up while the rider is still pushing down.

You can get a similar effect on a regular bike, still pushing down when the pedal hits the bottom of the circle.

If you start bouncing, slow your cadence a little, and work on a smooth pedal stroke. That's good for efficient riding, too. With practice, you can raise the cadence a little at a time. I don't pull up on the upstroke, except on hard sprints. At the most, I unweight the upstroke.


Rollers
The "rollers" advice: you have to be smooth on rollers, otherwise the bike won't stay on top! Any bad balance or awkward pedaling will affect the bike's location on the rollers.

First time on rollers youtube