VHP, BJD and EDL are products of Political Parties using Vote Bank Politics, Scared to take action against one group and allow extremism to build in society.
In West Bengal those Jamatis caused trouble, burnt down Hindu homes and all, and state govt. kept silent because of votes.
This gives support to organization like VHP BJD to retaliate in similar fashion. Hence you have riots.
Riots is just the result of Governmental failure for a long time which keeps heat to anger of the people.
But why political parties need them because Muslims hate non-Muslims and they get reaction then born BJD,VHP,EDL then Political parties Use them some way ! why jamatis burnt down hindu homes ? is simple their religion guide so ! their scripture guide so .....and so on reaction born . then parties comes in ground.