It's from Nexerelin.
Essentially in that game factions can capture other faction's markets. The Luddic Church fleet is (I think) an invading fleet. The invading fleet size is proportional to the thing it's invading, and the size of the market where the fleet is coming from. That fleet is from a very big market, going to invade another very big market. The pirate fleet is the type that spawns when an invading fleet attacks another market (hence, 'Response' fleet). Normally the response fleet and the invading fleet clash, and from the outcome of that battle the market ends up changing hands or not. Afterwards the fleet despawns. However, sometimes either fleet can end up getting distracted, chasing random enemy boats until they end up somewhere else entirely (See my earlier picture on the Hegemony Response Fleet).
Alternatively, that Luddic fleet is actually a hunter fleet - I'm not sure if Purification means 'hunter' or 'invader'. Hunter fleets spawn when you get to Vengeful against another faction. They end up sending fleets at you proportional to your own fleet; so if you have a scary fleet, the enemy will send scary fleets at you. Not sure if that's SS+, Dynasector or Nexerelin that does this.
Also, if anyone was wondering - The Sunder-D has managed to outlive all the Sunders. I am not joking. I have not needed to replace it or re-pick it up after battle, it just refuses to die. It's obscene that the half broken, piddly little (D) version lasts longer than the friggin over-worked, hot-tuned Fracture in heavy combat vs. Templar fleets.