Have you tried again and check if it does that again ?
If it does, try attacking with different skills… or a different weapon (ie, a non-ethereal one). I suspect you’re doing too much damage… much like the wizard with black hole. If you’re one-shotting everything in sight, you might also want to try increasing the difficulty and see how that works.
I’m not familiar with the Necromancer… but what you describe is similar to what we have been seeing for the wizard, and now seeing also in other classes.