if you turn your coat then you show your back... and one who back stabs you is a traitor..
u have a coat with inside having indian colours and outside having pakistan colours. a traitor will possess such a coat
A TURNCOAT is someone who FLUCTUATES - A person who quickly changes sides in a dispute etc.
Read it as Changing the Coat or Uniform : A traitor changes the military uniform and wears the enemy uniform ( metaphorically)