I know, in humans, if we have a lot of sugar in our system it will cause our pee to bubble more (when it hits the water in the toilet). I know bird waste and human waste are nothing like each others ... But, if sugars can cause our fluid waste to bubble, it stands to reason that it COULD cause the liquids in bird waste to bubble too ...
Is the bubbling associated with any food changes or special treats you might give?
I have no idea about the food changes associated with the bubbles. I try new foods all the time. I did make a mash the other day it has some beans in it and she did have some today too.
Diet impacts all poop and pee, so if you've got them on a really varied diet, things are likely to crop up as 'weird'. Wait until you see the outcome of them eating all red food, you'll think they've ruptured something!

As long as there's poop in proportion to the food they're consuming (and they're fine otherwise!), generally all is well!