The galaxy can never sustain peace for long, but this is how the current events are known to have started. Every single habitable planet was colonized to some extent, and population numbers were through the roof, and everything was for the most part peaceful and trading was at the all time highest as well thanks to the resource accessibility from expansion.
The outer ring planets is where civilization all started, it grew, and grew, and grew. Expansion to other worlds would happen as fast as technology would allow it, which was fairly slow, though filling the entire galaxy with humanities small colonies, they all thrived so well due to the untapped resources everywhere giving tons of jobs and economic opportunities, The problem though, resources were all depleted back in the home systems, and only so much migrating can happen to the newer colonies that aren't developed enough to hold more people. this caused a large overpopulation of the home systems, which then lead to rising living costs, and ultimately a massive growth of the poor population.
Colonies were for the most part, still loyal to the home systems, but the home systems were not loyal back because they were not reaping the benefits to the majority. This began to cause a divide and disruption to political discourse. Factions began forming to counter each other, not just 2 sides, the populous really couldn't agree on any proper way to fix the system, which lead to civil war, and then eventually colonies taken over by many different factions. These factions were not going to discuss in a good political manor, they started to use threats, then by force.
Two factions really grew the most powerful, where the smaller ones mostly were not worrisome, or were loyal to either of the 2 strongest. These 2 factions battled in small disputes at first, but it ultimately lead to every habitable planet getting bombarded to the point of inhospitable living conditions. This never lead to a stable government anywhere since.
This event is known as the great planet collapse. There was no large military or government that could protect them, because all the factions in the political environment were in constant disagreement and war. Survivors of the bombardment mostly just stole or built junked out space ships to live and roam in while trying to not get blown up.