Why are crackers burnt after a person dies in Hindu religion?
First of all, use of crackers is not mentioned in our scriptures. Not every caste in Hindu religion burn crackers after death of a person. Crackers are burnt (now a days) during happy situations and during to celebrate anything.
When we come to your question, people also play some music while taking the dead body to the burial ground or cremation ground. It is done so to indicate that the person who has passed away was a great man and lived a great life and to celebrate his detachment towards worldly pleasures and journey to moksha. Not all the Hindus use crackers after death. It is just a local custom. Brahmins do not burn crackers and not even use music while heading to cremation ground. Instead they chant the name of Govinda Govinda.
By this we can conclude that using crackers or music after death of a person is a local custom .