Black orbs that give Sigil Shards

Went into the Nether a few times (Explore), and once I saw black orbs dropping that give 3 sigil shards each when shot. They have the "kill enemies" patrol icon on them when you get close. There were a lot of them just lying around at the spawn in location, more dropped later from enemies. Got me to 300 stones in no time.

Didn't happen in any other run. Anyone seen this? Do you know what triggered this?