The theoretical world of Hēdran is the 2nd planet of a 7 planet system. It lies within its star' s (G1.4) habitable zone, but with a hot jupiter gas giant only ~1 AU away it experiences heavy gravitational flux (this will be important later).
Hēdran is the remnants of a carbon planet and as such it is already rich in hydrocarbons, but for the majority of its life is has been an oxygenless barren world. Until planetary drift occurred bringing one of he gas giants to the innermost orbit. Hēdran was spared annihilation but was instead seeded with liquid water and other chemicals necessary for life.
Now eons* later it maintains an atmosphere slightly different to our own
As mentioned before the planet receives more than the usual amount of gravitational flux due to the close gas giant. This has caused an increase in volcanism and geological activity. This is how I explain the replenishment of gasses such as SO2 and N2O, also a unique property of the planet is that it is inherently high in metals.
How these conditions might affect life will come in another, later, question.
Main question at hand
It is planned (though subject to change) that the bioavailable heavy metals (Platinum, Palladium, Chromium...) has led to the ability of complex life to be able to catalyze normally toxic gasses for use in biochemistry. For clarity's sake, yes photosynthesis and respiration will take place to replenish and reuse atmospheric gasses.
Given that life found a way to catalyze and use the gasses listed are there abiogenic reasons why my atmosphere would be unstable and eventually decay?
*Using Eons as a euphemism for an indeterminate passage of time