Higher velocity implies low pressure. So in whirlpool the velocity of water have to decrease with radius in order to have force towards its center.
My question is how is explained whirlpool formation in simple words.
A fluid motion in a vortex creates a dynamic pressure that is lowest in the center increasing radially ($P \propto r^2$). The gradient of this pressure that forces the fluid to rotate around the axis.
This is usually represented by a vector called vorticity, and defined by $\omega = \nabla \times v$.
In simple terms, this means that the fluid is rotating around a certain point. If you placed a small ball on the flow, you would observe that is would rotate about the center and the direction of vorticity vector is given by the right-hand rule.
The formation methods are many. For example, in the wake of an engine, there air has been given rotational momentum and will continue to have vorticity. When two opposite flows meet, they can also form as in planetary systems, like tornadoes or Jupiter great red spot