When a relic casts a specific spell, like bloodseeker’s blood burst, you gain infusions. Relic casts don’t trigger other manual cast effects, like pyromancy, so I think this is unintended.
Before and after screenshots:
Here I have the initial 5 random infusions. Zonte casts a single chaos spell (I disabled the perks which would cast additional gems, for testing). Bloodseeker casts 3 blood bursts, generating 3 infusions.