Bug or feature? Ships with very low HP explode instantly after they are deployed, during the burn phase. Happens especially with ships that were disabled and then repaired after an engagement. Happens to enemy fleets, too.
I wasn't able yet to check the HP of the damaged ships before deployment, but they had a few days to repair, so it should not have been zero from the start, I believe. Also pretty sure it's not asteroid impacts.
I'm running Starsector+, in case that has anything to do with it.
By the way, if you're in a battle with a big fleet with generally low CR, the low CR warning for every single ship gets annoying quickly.