There's a few factors that come into it. As Slide said, injuries/suspensions play a big part (which, at this stage has helped us with all our guns coming back at the business end). Origin I think plays a part as well, with the intensity it requires most players involved seem to go 'flat' for a few weeks after. Finally, and most importantly, the coaching staff ability to cope with all these factors. Each year we improved under Des, and I noticed the team seemed to 'peak' a little later each year until we got ot right. He and his coaching/playing staff tweaked things each year based on the previous years experience.
Some coaches however, don't seem willing/able to learn. Look at Brian Smith. When he has taken over teams, he has a reasonable record of getting to the GF, but his teams always seem to be dying on their run and fail to come up with the intensity required on the big day. He then, of course, stays to long, gives everyone the ****s and the team plays like busted's year after year.