I think on live servers DK heavy attacks get an executioner bonus.
Champion points into weapon attack and there is a passive in the middle blue star after 75 points that gives extra damage to low health targets with heavy or light attacks.
Couple that with DK buff abilities for heavy attack and the debut from focused aim and you get a 26 K heavy attack.
From max range in Cyrodiil, a fully charged Heavy Attack will land after a Snipe even if you start the Snipe after the Heavy Attack via animation canceling. ...or it was the other way around, the Snipe first, Heavy Attack second... Either way they land at about the same time from max distance. So it likely got the sneak attack multiplier, the Focused Aim armor debuff and potentially Molten Armaments though there is no evidence to indicate the attacker was a Dragonknight. Also, Heavy Attacks hit harder than everyone gives them credit for. With 75 points Blade Expert, Heavy Attacks hit harder than Puncture, almost as hard as Low Slash. With 100 points and Dragonknight Igneous Weapons, Heavy Attacks hit 15% harder than Wrecking Blow - and that is not from stealth.
"A person is smart. People are dumb, panicky, dangerous animals and you know it."
Focused Aim does help it but there's more to it than that. Bosmer or Khajit DK(10% from stealth), Maximize your weapon damage, split CP's between Mighty and Bow Expert, Major Brutality(20% weapon dmg), Molten Armaments(40% HA+150% if target is below 50%), 2 CP into Piercing with Sharpened bow.
I don't even have as high of weapon damage as some as I do not have a Maelstrom bow. It won't quite hit 26k though without you having low armor and below 50% health to start with. I wanna say its usually around 14-17k ish for me depending on targets armor.
Legendary Archer of Valenwood
Bosmer Dragon Knight Archer. XBox One. (Flawless Conqueror Bow/Bow)
Bosmer Nightblade Archer. Xbox One. (Flawless Conqueror Bow/Bow)
Bosmer Sorcerer Archer. Xbox One. (Flawless Conqueror Bow/Bow)
Bosmer Warden Archer. Xbox One. (Flawless Conqueror Bow/Bow)