your discs are like shock absorbers. when you fuse some the other discs must take up the job of just fighting gravity and the daily abuse that we give our backs from routine activities. it is not uncommon for this to happen.
can lumbar fusion cause problem for the disk above. i had l4-5 l5-s1 fusion now l3-4 is bulging. my md called this a cascade effect and is not surprised. i cannot find anything supporting this cascade effect.