battery drain

can any body tell me what would be the main reason for battery drain

Anything electrical. Either work your way round with a multimeter to find where the drain is coming from or get an auto electrition to check it for you.

Every year when it gets cold people start getting problems with batteries draining. Not necessarily the car but more likely the battery. A cell fails by the plates disintegrating and bits shorting out. That cell will drain the power from the healthy ones. A hydrometer check of the electrolyte will soon prove it.