Question

In a doble displacement reaction
AB+CD gives AD+BC
Then why does again B displaces D and it becomes AB+CD? Or why don't the reaction reverse?

Answer 1

The double displacement reaction happens because generally the products are chemically more stable than reactants. The have stronger bond, and in the process they may release energy and the system goes to less energy state. Most of the time products are more neutral that product.

Normally the reverse reaction doesn't occur because then the system have to go from lower to higher energy state.