I have fought a bunch of battles against Epic+ monsters since this thread was started, and have noticed a few strange deaths by thorns myself. my main heal is +250 HP on hit, and usually I win the battle, usually with half of my HP left. but there are a few times where all of a sudden I am dead, even when I am not near the monster. even an epic+ slime has taken me out when I am not close. so I wonder if Epic+ monsters have a damage boost to thorns? also, if a mob has thorns, and you hit the mob with an AOE attack, you get hit with multiple thorns and can die that way. with Hunter Perk, I have had 2+ Shrine Guardians appear with thorns, and when both their thorns are active and I am hitting them both at the same time, I end up dead if I am not careful.
one thing I have noticed is when my battery is low, there is a little lag, and I have died by thorns that way a few times, but there are still the other Death by Thorns I have experienced.
these are just some of my observations, I hope they are helpful.