That's intended behavior - which is why Zarys keeps shouting from above not to carry open flames around the place.
If the oil is set ablaze (even accidentally without any of the barrels exploding) they'll all retreat to the back of the cavern and become permanently hostile to protect their valuables. Nothing can be done with them anymore after that point.
You were right

It took too long, but I finally figured out the reason (even though I'm writing this only now, I couldn't do it before): my friend, the second player, had a torch equipped in his melee weapon slot. I didn't know about it, and I couldn't figure out why the explosion happened anyway, since he had a CROSSBOW in his hands when he entered the cave. So, the torch, even if simply inserted into the weapon slot, still sets fire to flammable surfaces. It was too unobvious at the time, but I figured it out when I "recreated" the situation in a new campaign to figure out the reason.
Thanks for the answers, all the best).