Okay Ive several theories regarding the Shivans. But I'll only post one for now
What the Shivans are
The Shivans are the Great Preservers. Or in MMORPG terms
Every Race are Players
The Ancients a race who went round commiting Genocide on a massive scale are PKers - Player Killers, players who goe round gratuitously killing other players.
The Shivans are the PKKers- Thats Player Killer Killer > Players who kill those who kill other players.
Shivan Culture
1) Shivans communicate through quantum wave pulses instead of Verbally. Verbal communication like humans and Vasudans use is the Shivans equivalent to using hand signals or Semaphore.
2) Shivans take insults of any kind very personally. If one shivan insults another, it results in a major blood feud. The losing side's forces and resources are absorbed or destroyed by the winning side. So one shivan never insults another unless he can backup his words
3) Shivans worship their Mothers. Shivan society revolves around their mothers. So never insult a Shivan's mother. When this happens it usually results in a holy war against the offending party. The only way to appologise is to offer the severed head and spleen of the offender to the insulted party.
The Great War
In their roles as the Universal PKKers, the Shivans observed the TV War. They chose not to interfere because it was looking to head towards mutual destruction for both sides. However, they kept the Lucifer on standby in case they had to act.
The Shivan observation fleet then encountered the GTI who eventually tried to commicate. Due to cultural and Liguistic differences resulted in the GTI effectivly "giving the Finger" or saying in Semaphore somthing along the lines of "Up yours, you s**t faced w***ers!" to the Lucifers. The result was one seriously pissed off Shivan Admiral who basically decided to avenge this Slight on their honour by saying "F**k observing i'm taking these SOBs out now." The Rest of the fleet Agreed and followed the Lucifer into battle. And we all know how that ended so you dont need me to tell you
The Second Shivan Incursion
After The Great War, Shivan High Command decided to leave the GTVA alone - they felt The spirit co-opperation between the Terrans and Vasudans when faced with anihilation would keep the GTVA way from Genocidal rampages and thus out of the Shivan's hitlist. The HoL and NTF wars were felt to be internal police matters and not worth the Shivan's attention. But that Changed when the BTC Trinity entered the Nebula
When the NTC Trinity entered the Nebula and attempted to communicate with the shivans in the Area via Quantum Wave pulses as is normal for the Shivans. Unfortunatly bad translation resulted resulted in the Trinity insulting the Shivans' mothers. In shivan culture insulting someone's mother is just about the worst thing you can do, so filled with righteous fury, the Shivans swore vengence and declaired Holy War against those who had insulted them so.
The signals from the SC Rephaim were challenges to Bosch and the GTVA to take back what had been said about their mothers. The Sathanas was also called in take capella hostage until the appology had been made. However it ran afoul of the Colossus.
Then bosch tried to forge an alliance. Unfortunatly in the Shinvan language, Bosch's message was the very worst insult imaginable. So bosch and his crew were captured and taken away for execution. By this time the Shivans realised that Bosch was a Renegade on the Run from the GTVA. So rather that take their Holy War to the GTVA, the Shivans instead decided to send them a message. However, since normal communication was impossible both ways, the shivans decided to resort to extreme methods.
The 111th Juggernaught Brigade was deployed to capella. The crew of the first Sathanas were also avenged when the Colossus was destroyed. The shivans then nuked the Stars. This was their way of saying "F**k off and leave us alone. This is what we are capable of. F**king well behave or we will do this you."