There's a game mechanic i wasn't aware of in x-piratez.
After following a medium ship , it landed and my Gals assaulted it, as nearly every medium on the dozens landing i attacked it was full of Ositron troopers and small saucers (sometime there's a larger old-xcom saucer but this time there was none) .
As the map was rather open and desert, i took it slowly (as Ositron opportunity fire can be devastating as they sport rather powerful weaponry) with using the Gals grenade launchers to eliminate those pests (while they're near impervious to our combat shotgun, those Ositron guys are usually one-shot by grenade launchers)
After clearing the exterior of the big ship, it was time to assault it as the game wasn't ending so i figured out there was more of those Ositron troopers inside.
Positioned everyone to rush into the elevator, turn 19 ended and boom
After googling a bit because i was wondering if it was just bad luck with an AI sporting some kind of nuke and accidentally shooting at his feet or if it was some stupid mechanic to force you to rush , it's the latter as the entrance in the bootypedia mention (too bad i didn't noticed first time i read it) : ships that are on supply mission will automatically explode after 20 turns (though it was the end of turn 19 for me) , probably to cut you from getting too nice loot once you have the tech to easily attack them (yeah sure)
Hurray for saves and NOT playing in ironman mode
Very oddly the explosion spared one of the Ositron trooper that was on top of the now utterly ruined ship, but the guy unfortunately for him didn't resisted a grenade landing right on his face.
Oh well, not the best loot but a nice score for the end of the month.