Well.. it took until now to get a response to the support ticket I made ingame regarding this. I didn't even know about this thread until now.
They were at the edge of our deed and I just wanted to catapult stuff they had built. At first I was just loading/winching the catapult inside the mine, dragging it outside and firing it. Couldn't do that anymore after couple hits because I got targeted (and you can't fire catapults in combat). I had to get creative. I logged in on another toon, dragged the catapult outside like I did before, but used the other toon to fire. At some point the second toon was targeted too. Well.. my idea at that point was that at least one of the toons should be out of combat at some point and I should be able to fire. I didn't realize it at first since you can't look at everything while dual clienting, but I realized that the catapult fired when I moved inside the mine (while being in combat). Tested it couple more times to try to understand what actually happened and Lisabet called me out.
And since I got asked to post required steps to reproduce this (two toons is enough):
Have a catapult outside a mine, load it with ammo, winch it, get another toon to attack you, right click catapult & select fire. Go inside the mine and profit.