It seems that the more you attack a planet with a Dark Spire Building on it, the more they respond. It gets expontential to the point that killing anything spawns a huge amount of Dark Spire ships.
Killing the dark spire ships doesn't feed into the loop, and a dark spire spawn is roughly 1/10th (iirc) the strength of the pile of ships that died to cause the spawn, so as long as the ratio of dark-spire losses to non-dark-spire losses (in terms of relative unit strength, not ship count) is higher than 1:10, the dark spire population on that planet will decrease over time.
However, something strange happens then that I didn't expect: Attacking other planets with Dark Spire Buildings also causes them to spawn on the previous planet I attacked.
Yep, when one spawner triggers, all other previously triggered spawners (wherever they are in the galaxy) get a half-strength spawn.
Or, to sum it all up: Chaos