Why did the Orissa cyclone occur?

All cyclones occur due to certain weather patterns, which causes them to form in essentially the same way.

Cyclones are caused by warm tropical moisture bearing clouds developing in open oceans or seas. Cyclones can only form over warm waters in the tropical regions of the oceans where the sea temperatures are 26.5 degrees Celsius or higher. They occur in areas of very low pressure when air that is heated by the sun rises rapidly, and becomes saturated with moisture which then condenses into high thunderclouds. As the atmosphere becomes favorable for development (no wind shearing in the higher parts of the atmosphere), normal thunder storms clump together.

When the hot air rises, cooler air rushes in to fill the area left vacant by the hot air. The Coriolis effect of the Earth spinning on its axis causes the air to spiral upwards with considerable force. This in turn causes the winds to rotate faster, causing the tropical low to deepen in intensity into a tropical depression, and eventually a cyclone which is anywhere between hundreds of kilometres to thousands of kilometres wide.

Cyclones are also characterised by strong winds, yet in their centre is a clear, calm region called the 'eye'. When the cyclone continues its course, and the winds return from the other direction, they may seem to be more violent. The winds are not just rotating; there is also the effect of the warmer air continually rising and cold air rushing in. That is why the winds are so strong, and seem to move in all directions.