In Borderlands 3, it's noted that any planet with Eridium outcroppings has an Eridian Vault. So, that begs the question of why Eridium shows up in Athenas despite the fact that there's no Vault shown or known to exist on the planet of Maya's birth. I'm of the mind that there is a Vault, but ancient Sirens opened it, destroyed the Vault Monster inside, looted the Vault, and sealed it completely.
Interesting theory, but there are a few holes in it. For one, Eridium did not actually begin appearing on Pandora until after the opening of the Vault of the Destroyer. While Promethea has had a Vault open before the Vault of the Rampager, Eden-6 has not. The Jakobs family has spent generations ensuring no would-be Vault Hunter could open the Vault of the Graveward because they didn't want to start another war between the Corporations. With this in mind, why does Eridium appear on the surface of Eden-6 before opening the Vault of the Graveward?
Another thing I would like to point out is I find it highly unlikely the Sirens would coalesce to open a potential Vault on Athenas, especially after they scattered across the six galaxies before Nyriad's return to Nekrotafeyo.
This is a bit of a stretch, but I think the most likely in-lore explanation for why Eridium appears on Athenas is due to influx of Sirens that flock to the planet, as noted by Typhon DeLeon.
"There's no place in the galaxy with more Sirens than Athenas. Must be somethin' in their water! Or Sirens were drawn to the place by some sort of... I dunno, call?"
- Typhon Log, "Sleezy Monks"
Finally, and an admittedly more boring explanation, is that the level designers wanted every Location in Borderlands 3 to drop Eridium in some capacity, whether it be from breakable Eridium outcroppings, Eridium containers, completing Optional Objectives in the three available Circles of Slaughter (Cistern of Slaughter, The Slaughter Shaft, and Slaughterstar 3000), or defeating Bosses.