In most cases, the structural movements of the houses cause the ceiling cracks. It could be a minor earthquake, shrinking of the components of framing or the drying up of the lumber. If there are large cracks, there could be a problem with the soul or the foundation of your house.
If there is a large crack where you can insert a quarter, you should get in touch with a professional for fixing it as it may be a result of the structural problem of your house. If there are small cracks on the ceiling there is an easy way of dealing with them. The cracks could be dilled in by paintable caulking. You could also some drywall tape of fiberglass and then make an application of drywall joint so that the finish of the ceiling is done properly. Hairline cracks are not something to be worried about and you can get them fixed with caulking any time.