They have never placed any value on engineering, have a founder leading the engineering effort who is a complete buffoon and pay 30-50% less than similar silicon valley enterprises. It's not really much of a mystery at all why they are down so much.
Compared to other social media sites, maybe because if there is flashpoint, everyone on the site can get funneled towards it? On Facebook or Twitter you wouldn't find everyone commenting on the same "thread".
Does reddit do post mortems? If it did then we might have a clue.