No mods were in play but I was either in or 1 removed from a gas giant. The one next to the gas giant was a space ship grave yard
Gas Giants have an ability called unstable gases or something that damages all nearby ships and strikecraft whenever one of them explodes. There is currently a bug where Strikecraft will not lose this effect properly when they leave the Gas Giant gravity well. Thus if you sent a unit with strikecraft through a gas giant, then at another planet one of the fighters dies, they will deal splash damage to all of its squad mates and the chain reaction can easily kill them all.
If you were actually in a gas giant then this is expected behavior, it is just strikecraft are hit hardest by the splash damage since they have low health. Even corvettes and LRFs need to be careful not to be too close to exploding units in gas giant gravity wells.
This is stated to be fixed in the next update, and in fact may already be corrected in the 1.82 beta patch.
