I was playing my minion necromancer and noticed that seemingly random I would become vulnerable and my health globe would look shattered.
At first I thought that maybe some random trap or enemy just hit me and I didn't notice it. But then in town I managed to get my minion army in a position where one of my skeletons would constantly try to teleport to get to me and get pushed back by other minions. I noticed that every time that skeleton teleported the vulnerable indicator on my health globe would refresh.
Later I also managed to reproduce this bug by going out of town so my minions would fan out, and then back into the town so my minions would teleport to me. Every time I went back to the town and minions teleported to me I got vulnerable applied to me.
I started to remove my skill points one by one, and after I removed the point from skill "Vulnerable", that gives skeletons warriors 30% chance to make the enemy vulnerable, it got fixed. After I put point back into that skills and forced my skeletons to teleport again I got vulnerable applied to me again.
So if any other necromancer player are wondering why they get vulnerable applied to them seemingly randomly, that's the reason.