[ZooL]

I really hate how there is no clear rule to how your stress decreases.
you go into cover and sometimes it happens really fast, sometimes it takes ages. I know that it is partly influenced by how many enemies are near by and shooting at your cover but in general this all seems way to random.
and dying because you did not degenerate stress for 10+ seconds, not knowing why, feels horrible.

[Level9Drow]

It's also the faction you play. Wardog faction heroes NEVER go down in stress (Alicia, Loath, H.I.V.E. and soon Rak Mayura).

[LordDraco3]

Stress stays up if you take damage or you run.

Walking will make it lower, standing still or going into cover will make it go down. But if you take damage or start shooting, it goes back up. I have not seen any randomness attributed to the stress meter.