[RedDevil]

The wheels going over a speed bump travel a greater distance than the wheels on a flat surface.
When they are not driven they temporarily speed up, forced forward by the mass of the car. But the driven wheels don't speed up much as they are connected to the engine, which tries to maintain its revs and has a large rotational mass anyway - so the car has to slow down a bit; especially on the upslope.

With 4WD this effect is spread over 2 axles, so when the front wheels hit the bump and slow the car the rear wheels resist the slowing down and force the car over, and likewise when the rear wheels climb the bump the front wheels will pull it over.

There are formidable speed bumps where my parents live, and when my dad bought a Subaru the way it kept going at steady speed over those speed bumps was the thing that impressed me most.

With a velomobile there is very little rotational mass in the wheels, even the driven one. If it also has smooth wheel travel it could float over speed bumps as if they were hardly there.