A contingency is basically a statement in your purchase agreement that you will buy a house if some specific conditions are met or some particular event happens. Some of these conditions are external to that property and some of them are directly related to the property.
If you are looking for the top reasons to back out of an offer, you have to check your contract and read the contingencies. Once you made the offer, the owner accepted, and the earnest money deposit was made, you became contractually and legally bound to buy the property. The results of the home inspection are also a top reason that is used to back out of an offer.