Whilst I am not aware of the finer details of how the TAB or Centrebet for instance calculate odds exactly, the short answer is that they look at a range of factors in determining the opening price for the season or for a single match. Past success for instance, and whether a team has recruited well in the off-season, any injuries or even off-field issues that may dampen morale or even public opinion.
After this point, fluctuations can also be influenced by the amount of money coming in for one particular team. If 75% of the money is coming in for the ousider, than the odds are going to be reduced for that team, and the favourites odds will push out a bit. A bookie generally wants to try keep the betting as even as possible, that is, have an even amount of money coming in for both sides. By moving the odds are changing the lines for instance they steer punters of a favoured team and onto a team with ‘more value’. Keeping the odds even reduces risk for the bookie, basically the losers money will be used to pay the winners money - there will be left-overs and this is what the bookie keeps. This is why public opinion may influence the setting of odds, because the bookie can assume that it will reflect the money coming in to an extent.
In relation to this years premiership odds - I guess they still think the Tigers look good at this point, better than the other teams with the same win/loss. That is a pretty fair assessment.