anyone know how to stop a controller from opening up steam?

Long story short, I bought a Retrobit N64 controller (shown below). It looks a bit weird but feels amazing in the hands. There's only one problem. For some reason, when I'm playing games on Retroarch, sometimes pressing A will randomly open steam on my other screen. Is there any way to stop this from happening?
https://gamefaqs.gamespot.com/a/user_image/7/8/3/AARNb5AADwq3.png

there's an option for that? I didn't know. I didn't even have steam booted up. I'll have a look later then.