The main cause of Cyclone Sidr was Bangladesh's location in relation to the equator, because it is so near to the equator the sun is extremely strong causing masses of sea water from the bay of Bengal to evaporate. The water then starts the water cycle, but when it gets higher up some of the water starts to descend again causing high pressure on the surface of the sea in a ring and low pressure inside the ring. This causes the wind to take a spiral shape.

Also then the cyclone is 'funneled

' towards Bangladesh by India & Burma.

Hope this helps.