It's nearly two years old.
I do have filter media ("biohome") that claims to harbour bacteria which turn nitrate into nitrogen gas. I wonder if some of these bacteria have migrated from media to substrate.
Having said that, as far as I've noticed, it's only started bubbling a few months ago.
Recent changes to regime are I removed DIY CO2 and started doing water frequent water changes. (Though I haven't done them for the past month, and there are still bubbles.)
My tank has in the past had about 10-20ppm of nitrate, though I haven't tested for a while. I used to hardly ever do water changes, until I found that doing them was the way to get rid of BBA.
When I first set up my tank I tested my tap water a couple of times. The first time (summer) it had something like 100ppm nitrate and the second time (winter) it had 0ppm.