Why is my steam deck increasing volume automatically?

Definitely sounds like the Volume up button thinks it’s being depressed even if it’s not. My first thought is a stuck down button. That could be just the button or the housing is misshapen in this area (because of a drop usually).

But usually buttons like these work by temporarily shorting the circuit to ground. So it could also be liquid damage or just plain electrical fault.

Having a look at a closeup of the board, I would test the button as follows (adding picture for context)

With a multimeter check to see if this point on the board is reading as ground (is it continuous or less than 1 ohm of resistance to the copper bits around where the screws fasten this to the chassis)? It looks like this is a test point on the line coming straight from this button. Does it change when you press the button? I would expect this to have a higher resistance reading, but to be short when the button is pressed. If it’s always short to ground then something on that line is short (my guess is button) but no way of knowing exactly what for sure because if the circuit is short to ground, everything on that circuit will read that way. No way to isolate that to the button without removing the button (and retesting).

As for fining a replacement board, or button, I found a reddit post where someone purchased 4.5 x 4.5 x 4.5 tactile buttons. The link is since dead, but let me see if I can find something. These look a little different than the ones used by the shoulder buttons.