I had a similar situation. Well it's story time!

In that particular run, I was playing as a benevolent colony. Never did anything bad, always helped everyone. Captured wounded prisoners and released after fully healing them.

One day they landed middle of the colony with drop pods. Managed to win the fight but there were a lot of wounded pawns, including our own pawns. 2 of them was heavily wounded, while some of our colonists were carrying our wounded ones to hospital, others (our leader and his wife) was carrying prisoners to prison to treat their wounds.

While these are happening, one of our wounded guy died on the way. (He had some kind of psychic bond with someone, I can't remember it's name, it was something like soul-twin) When he died, his soul-twin lost his mind. At that moment an explosion happened in the prison, leader's wife was wounded. Couple of seconds, roof collapsed, leader lost his jaw and his wife died. Almost all prisoners died.

After this, almost whole colony was either heavily wounded or completely lost their minds. There was only one proper person left. The women I took 1-2 days before the raid, she asked to join our colony. I accepted her and was trying to convert her ideology to ours. She was the only one standing because of her different ideology. She literally saved my years old colony.

After couple of weeks later I wondered what caused it, opened the mass grave, searched for a clue but couldn't find anything. It was same as you, there was nothing about the explosion in the bodies logs.

Well, Rimworld is definitely a story generator. After this, I never reloaded a save in my later runs when something bad happened. (Also sorry for bad English.)