Yeah, there were a few days between this feature being added, and the ability to toggle it in the settings... all just part of the growing pains with these new apps being developed and growing with the platform :)
When I notice that something isn't working quite the way I want, I've gotten in the habit of 1) checking if there is an update available which might have addressed it, and 2) poking into the settings menus to see if a new feature has been added that changes the look/feel unexpectedly.
As long as you are on version 1.0.96 or higher, you should have an option in the settings to turn off volume key comment navigation that should fix your issue.
Whoa. That should not be on by default. I'm all for experimental UI but I doubt many users will know what's happening with their volume. Apps should not be remapping hardware buttons in unfamiliar ways.
I've had several instances where a post hit me with surprise porn (that's somewhat on me for not noticing what community it was from on c/all) and then people are around hearing stuff. Cute me panicking when trying to shut it up.
Or even an innocent video that I can't hear. Definitely never change volume buttons by default without any warning.