I'm thinking this is either related to recount or omen. I have both installed right now, when my game starts my ui usage is about 40mb or so, by the end of the raid it shoots up to over 70. This just started happening with the addition of these two mods but I think it might be recount related by I could be wrong.
I noticed similar behavior last night in Karazhan, though not quite that bad. Roughly 4 hours translated to a 40mb -> 59mb usage. I suspect if I'd had Recount for Gruul the night before that I might have seen a larger jump due to more players/data.
I'm gonna guess it's recount - I've been running Omen for a while yet (obviously) and haven't had garbage inflation for a couple of weeks now. Recount, on the other hand, stores a lot of data, which makes it more suspect.
Easy enough to determine which - disable one, play for an hour, and check your usage. You can also use FuBar_PerformanceFu to eyeball 'em.
Recount is very, very heavy on the memory usage due to the sheer amount of data it stores. I've never noticed it causing performance problems even when PerformanceFu reported it using 20 MB of data however.
I'd recommend Violation instead if you don't care about the amount of detail Recount provides, it's far lighter on the memory usage.
Its the time based data in Recount causing the memory inflation. I advise you turn off time based data recording in the filter options if it really bothers you though I doubt you will notice any performance differences except when working with the Graph data (the more time based data thats recorded the more work it has to do of course). Shortly autodeletion of old time based data should be going which will autodelete data older than a set time period.