I don't know what exactly would be causing that. My 'nearest' guess is unlikely - that there might be a conflict with the Mac audio settings. I haven't heard of it changing mid stream though. I have talked with a few people on here who have had an issue with audio which turned out to be that their audio settings (outside of Pod Farm) were overriding their ux2/Pod Farm volume or input settings. I'm on windows myself but would be interested to know if anything strange happens in your mac audio settings area when this happens. Probably not that but one thing that might be worth ruling out (apologies if you have already had a look). (Actually, now that I think of it, I am wondering if it was changing mid stream for them - can't remember)
Another thing I can think of is if something has been automated to drop the level when something happens or there might be a 'midi learn' thing doing this. I don't think I've used 'midi learn' to work with my music keyboard but you can use this to allow your keyboard or other midi device, to trigger settings to change in Pod Farm.
You didn't mention you are using a DAW/recording program, but if you are using pod farm in a daw/rec. program, then maybe something has been set to automatically change the level. Unlikely, but worth checking maybe.
To check for midi triggers, have a look in the assignments view in Pod Farm. I am looking at mine and I have Tone A and Tone B levels in there and both set to 0.0db max level. If they are in there (and you don't want anything to control those levels, go back to mixer view and right click on the vol sliders and click clear (I have just done this myself - I must have at sometime
Also, when right clicking on the slider, there is an option like 'set current level as max' or something. Is it possible that its been set to go to halfway as max? Maybe try raising it to 0.0db and click the 'set...max.' thing again and see if it helps. Again, apologies if you have tried any of these things - just wanted to mention anything I can think of.
Maybe someone else has some ideas.