Misfiring.

This was done for a 1997 Plymouth Grand Voyager---Replaced spark plugs on the front of the engine, even numbered plugs, ran van for a mouth. All was well. Changed the two odd number plugs in the back of the engine, now engine backfires, appears almost like a timing problem. What would cause it.(Same plugs and wires in the back as in the front). What was done wrong? Please reply.