Girthiness can be caused by poor handling (rushing tightening the girth, etc). Some horses develop it as part of an overall objection to being ridden - either because they are bored, lazy, or fed up with their rider.
It can also be a sign that the saddle is not fitting correctly. Girthiness can also be cured by changing the girth - in some cases I've seen it fixed simply by changing girth material or changing to a girth with stretch panels in it. Some girthy horses start to behave if an old fashioned string or rope girth or cinch is used...many horses seem to prefer these even though some kinds look like torture devices.