it is possible that the timing could be off. also, the plug wire and the plug may be shorting out. check flywheel key to see if it is loose or sheared.
Not knowing the model of car or the engine service history then it is very hard to say for sure. In most cases backfiring is either caused by a timing issue or by an over rich condition. Make sure your engine is correctly timed and give it a good tune up making sure to pay attention to the plug gaps.
Backfiring for almost any car is caused by nearly the same factors. These factors are as follows: a lot of excess fuel having been built up in the exhaust system, late ignition timing, a choke that is not operating properly, malfunctioning distributor cap, bad spark plugs, etc..