This is not a bug.
When a dune dervish is spinning, they are shielded (so take no damage) and also reflect projectiles back to players. As a well geared Impale DH, you’re throwing daggers capable of doing trillions of damage. When these are reflected back at your hero, usually with a health pool of under a million, even with shields / mitigation up, your own daggers one-shot you.
The reason the death text says “Heroname was slain” rather than “Heroname was slain by cause” is because the game was only coded to give death messages when the hero was killed by a mob or the environment. When you kill yourself, it doesn’t know what the cause was, so it cannot say.
This can also happen in parties, i.e. other players’ projectiles can be thrown back against your hero, and your projectiles can be thrown back against them. Another hero with this sort of issue is the H90/Frenzy Barbarian, as they use a belt which grants all runes to Frenzy, which includes Sidearm, that causes throwing axes to be cast whilst attacking.
Worst version of these to come across is if they have missile dampening. That way, you could have huge numbers of projectiles going into the pack and, when the big elite dies, the dampening drops, and huge numbers of projectiles come back to the party.
The short version of this is… if you’re playing an Impale DH, you skip dervishes unless you have a shield pylon active, or you will 100% kill yourself.