Forced marriages happen for lots of different reasons.
In some cultures it's traditional for the parents to choose a husband or wife for their child because they feel they know what is best.
Some parents justify the marriage as protecting their children. Other families feel that their honour and reputation is at stake if they don't choose who their child marries. Sometimes marriages are formed for financial reasons.
Is it religious thing?
Forced marriages are not a part of a particular religion. Christian, Hindu, Muslim and Sikh marriages all require both the bride and groom to fully consent to the wedding.