Question

some space on top of water tanker left unfilled by water during filling the tanker.Why?

Answer 1

When the tanker comes to rest from motion, the water is still at motion due to inertia of motion. That is why, the water splashes forwards.
When the tanker starts moving from rest, the water tries to remain at rest due to inertia of rest. That is why, the water splashes backwards.
If no space is left, the water may overflow. That is why, some space is left.