Simply imagine what sound a 3D-object of the shape as in the WP-article would make.
The star-shaped object would make a "spiky" sound, the bubble-like object more of a bump-like sound. That explains the attribution of the words kiki and bouba to the objects in different cultures. My explanation.
Make a sound how? When dropping the 3d object onto the ground? Or when making it resonate at its natural frequency, or something else?
What would fit GP comment is: spin along the central axis perpendicular to the plane of spikes/bumps with a constant angular velocity.
The words spike, bubble and bump seem to be examples of this effect, not really explanations of it.
Yeah it seems kind of obvious really. But still interesting.