I think that super rugby franchises are allowed to name 28 players that they want first dibs on from their existing squad or their feeder NPC teams, after that it's pretty much a free market. I think NZRU control the pay and contracts for all professional players; so players outside of those 28 tend to go where they think they'll get most game time.In NZ, it appears that player turnover is also driven by the NZRU allocating players
Like Aaron Smith from Manawatu couldn't get into the Hurricanes roster due to Piri Weepu, and didn't want to be second place like TJ Perenara, so he went to the Highlanders.