UAD models hardware stuff, which is sometimes very susceptible to the amount of time it's being used and sound different just a couple of minutes after they've been turned on and even then, components shift state and respond different when being fed signal for a while. I know the LA2A model in Digital Performer 6 needs warming up because of this, they wanted to model it's behavior just like it would be if you had one in your rack, and they setup a function so that you could record the input stage at any given time so you wouldn't have to wait for the whole song to play just to listen to how that particular section would sound on mixdown. But if you reset the plug, (by disabling the track or messing with buffer settings or whatever) you need to "warm it up" again. Maybe... just maybe, UAD is doing something akin to this with some of their plugs. It would make sense.