Either Davy Jones or Imu's Domi Reversi'd Rocks.
Since Haki is the biggest thing in the series, I think it's tied to Haki: Imu didn't use Domi Reversi on Brogy right away. Instead, Imu blew off Brogy's hand.
Why?
Because, Haki is Willpower, and when Brogy was furious with Imu, Brogy could resist. After Brogy's hand was blown off, and the rest of the Giants saw him turn into a murderous demon? Bam, everyone gets Domi Reversi'd.
I figured Rocks couldn't get Domi Reversi'd here, because of his gigantic CoC Haki.
But, if Rocks found his dead wife... and saw the person (I think it's Garling) who killed her... Rocks very well could have been Domi Reversi'd. If Domi Reversi works on people who have lost their Will, Rocks seeing his dead wife and wanting to destroy the person who killed her is a very legit reason he could be Domi Reversi'd.
Either Davy Jones or Imu possessed Rocks after his wife died.
This fits well thematically with Harald: like Rocks, Harald was killed after Imu Domi Reversi'd them. Elbaph hates Ida, but loves Harald: if Ida died, not many people would honor her death, sadly, the way they do Harald.
I think Harald and Rocks both got Domi Reversi'd for the same reason: they watched their wife die. I'd imagine Harald keeps a Den Den Mushi on tap to contact Ida. This is what I think went down:
-The WG believes (correctly) that Harald jobbed to Rocks. We saw Sommers talking about how everything was Harald's fault. I think Sommers' child died on God Valley: Harald was supposed to kill Rocks, and, if he did, God Valley doesn't become what it did. I believe "God Valley" refers to Uranus, and also the Celestial Dragons who died there, including the un-named Gorosei. Sommers is PISSED here: I 100% believe he lost a child on God Valley, and that's why he sadistically uses his Fruit to trick kids into murdering their parents. To make children suffer in the name of his dead child. Rocks warned Harald the WG just wanted to use them as military slaves, and Sommers just admits that here.
-Shamrock is said to have known Harald, and Sommers meant Shamrock visiting Harald's grave. I believe Harald convinced Shamrock that other races can also be strong/noble. Garling brought Shamrock to Elbaph at some point, when negotiating with Harald after God Valley (Harald still wanted to join the WG, but change it from within, like the Marines do). I believe the Abyss can travel where you've been before, like Kuma's Fruit: Shamrock could travel to Elbaph's Throne Room, because he visited King Harald alongside Garling. This also means Garling can probably teleport there, and could show up soon. Also likely Shamrock, Shanks, and Garling all hung out at the Reverie Shanks mentions meeting Harald at.
-Garling/Sommers come to despise Harald for pretending to want to join the WG, then betraying them by helping Rocks. I also believe Harald went to God Valley (possibly with Loki sneaking aboard) to save the people of God Valley/Slaves. Garling and Sommers plot to take down Harald with Imu.
-Sommers murders Ida (likes killing parents), and Harald is made to listen to her final moments. Destroyed by grief, Harald, like Rocks, and the Giant Warrior Pirates, gets Domi Reversi'd by Imu
-Jarul holds Harald down while Loki stabs Harald, because he's too strong to be put down normally.
-Harald shrinks down back to normal as he dies, and Jarul gets memory loss from Loki impaling the sword into his head when he stabbed Harald. Harald shrinking knocks the sword out of him.
-Shanks and Gyaban come in to see Harald dead with Loki covered in blood surrounded by the rest of the dead Giants
Ida was murdered by Garling/Sommers, to break Harald's Will, and allow Imu to Domi Reversi Harald. It's possible Harald got a Mark of the Abyss, but, since I think he broke his horns off after seeing Domi Reversi on God Valley with Gunko, I believe Harald was more wary of the WG post God Valley.
If Imu breaks your Will entirely, Domi Reversi goes through.
Rocks got Domi Reversi'd over his wife dying, and was killed by his friend, Harald.
Harald got Domi Reversi'd over his wife dying, and was killed by his son, Loki.
Oda popped off like the finest champagne this arc, my word.