RB + A toggles between fullscreen and windowed

[LunaMoo]

There's no such functionality in PPSSPP, unless you use extra software or system function that causes RB+A to be registered as alt+tab or double click, it should never happen. Alt+tab is standard windows hotkey so you can't do anything about it, no idea if other systems have something alike, double click changing between windowed and fullscreen can be disabled by editing PPSSPP ini file manually(the option is calld FullscreenOnDoubleclick), or if you want to quickly check if that helps, by activating touch controls(you can even hide all the touch buttons, it's enough for it to be enabled) or in case of windows enable mouse control, this also force-disables double click fullscreen switch(otherwise people using mouse would experience what you're experiencing).

Either way it looks like the root of the problem is something on your system as you're the only person affected.