The first that would be bumped would be those who have not had a cabin assigned to them. (Guarantees)
I actually believe that a new cruiser would have less of a chance of being bumped than a repeat passenger. They know that if they bump the first time cruiser, on that line, that person will probably never book with them again unless the compensation is good enough.
Long time cruisers already have some built in loyalty and may have a greater tolerance.
It's like upgrades. It "seems" like more new cruisers are given upgrades than repeat cruisers.
This is just my opinion, based on years of cruising, talking to many long time and first time cruisers and reading thousands of post.