how did communal politics emerge.

It mostly emerges in context of the narrow and vested intrests.Some reasons can be as:

• Communalism can take the form of our daily beliefs. This involves religious prejudice, social stigmas and superiority of one's religion. 
• A communal mind may lead to some unjustified political behaviour. Vote bank maybe developed on the basis of religion. 
• Communalism maybe used as a weapon to promote a political party. This involves use of religious symbols or some famous religious personality.
• The most ugly form of communalism are riots. Riots during partitions is one of the biggest and ugliest example of communal riots. Similarly, the recent Muzzafarnagar riots are also an example of communal riots.