There are an amazing number of reasons that can cause the ceiling to start cracking. It basically depends on where your house is located, how old the building is and what material was used in building the house.

Some cracks in buildings may appear due to improper trusses used in the construction the house. If you are living in a flat, it can also happen that the cracks in ceiling have got something to do with what the family upstairs is up to. If they are going to have a clogged bathroom, the excess moisture can also moisten your ceiling causing it to crack.

In my 35 years of experience in the real estate industry, I find that most cracks in ceilings and walls are due to settling.  Many times homes are built on ground that has been moved and compacted.  As the home ages, it has a tendancy to settle.  As the home settles, cracks appear. 

Make sure you have the home inspected by a reputable home inspector or building engineer.  They will be able give you a better idea of why the cracks are appearing.  If the cracks appeared years ago, they probably aren\'t moving any more.  However, the danger is that if the home is still settling it would be cause for alarm and potentially costly repairs.