Why the Inrush current increase with decreasing voltage?
My question is why the current is increasing when the voltage is
  decreasing


Current rises at a rate determined by voltage. If the voltage is positive and small in amplitude, current rises at a small rate. If the voltage is positive and large in amplitude, current rises at a large rate. Current rises and continues to rise when a positive voltage is applied across the inductor terminals even if that voltage is falling towards zero. The current seeks to find a value that represents the "area" of the voltage waveform and, only if that voltage fell to 0 volts will the current remain constant as per: -

$$V = L\dfrac{di}{dt}$$

With the voltage now at 0 volts, there is no area under the voltage waveform and hence, there is no need for the current to increase or decrease with time i.e. it remains at a constant value.


  Its obvious that current should be increasing with increasing voltage
  and current should be decreasing with decreasing voltage


No, that's not how it works. 

Consider the case that the voltage is negative but rising towards zero volts (i.e. an increasing voltage) - the current will continue to fall negatively while the voltage is rising towards zero volts. Only when the voltage reaches 0 volts will the current stop changing. That is because the slope of current is determined by the applied voltage or: -

$$V = L\dfrac{di}{dt}$$

Simple mechanical analogy

You have an object that can travel only forwards or backwards. 


If the object's velocity was +1 m/s then after 10 seconds the distance traveled will be +10 metres. 
If the forward speed slowed, the object's distance from the starting point would still increase even though the object was slowing down. 
If the speed fell to 0 m/s then the object would still be at a positive distance from the starting point and, that distance will remain fixed until the object started moving again. 
If the object then moved (backwards) at -1 m/s, then the positive distance would start to fall towards zero.
If the object continued to move backwards, the distance from the starting point would eventually become negative.


In this analogy: - 


Velocity behaves like inductor voltage
Distance behaves like inductor current