Yeah I think it just depends on if they have ROFR "Turned on" for a certain resort, and if it's below a certain price point. Some times are almost totally safe if they have it turned off, and then the lower priced contracts are susceptible if it's turned on for a certain resort at that time.
The graphs make it pretty easy to see. There are small periods with a lot of purple ROFR when they want points, always towards the bottom of the graph, then they turn it off for a while. I wonder if it just has to do with how many standing add on requests they have for sold out direct points that they think they can easily flip.