Pressure.
If you pass over anything like plaster or mortar (lime or cement) applying very light pressure the solid particles are pressed in and the water rises to the surface as it's displaced from within the mix. A flexible trowel can't apply enough force to keep the water from passing beneath the blade so it stays on the surface.
If you do the same but apply strong pressure then the same happens, but the water forms a 'bow wave' in front of whatever you use to apply the pressure. That bow wave is then pressed back into the mix as the trowel coming behind prevents it sitting on the surface.
If you want to prove this, pass a steel trowel over plaster (or mortar) but just lightly touch the surface and it will produce the same effect as a flexible trowel.