Short version: they were engineered under the Iconians to disrupt our galaxy for their return to conquer it

Long version: The Iconians have been slowly manipulating galactic affairs because their race was almost obliterated 200,000 years ago and their cumulative knowledge stored on a device called the World Heart was stolen by the Other so they plotted revenge.

Canon events Iconians were responsible for, either directly or by proxy…

1. The Hobus supernova which destroyed Romulus

2. The bluegills from Conspiracy, engineered by the Elachi (Shroomies) from ENT Silent Enemy

3. The Solanae (clickers) abducting ENT-D crew members and experimenting on them in Schisms

4. Building the Jenolan Dyson Sphere from Relics

They were low-key allied with the Tal Shiar who was still trying to maintain control of the Romulans, which lead to some agents, including Sela, having access to some iconian gateways. They also were impersonating Alpha/Beta quadrant powers into attacking fluidic space which provoked species 8472 (Undine) into attacking, leading to a breakdown in relations between UFP and Klingons, but also escalated the Borg’s plans to assimilate 8472. While helping Romulans find a new home planet you find a large gateway which leads you to another Dyson sphere where the solanea were experimenting on others, but more importantly where Omega particles were being generated. This opened up a direct path to the Jenolan Dyson sphere which had “jumped” to the delta quadrant and the Vaadwaur have returned with a vengeance and modern tech ultimately because most of the higher ups were under control of the bluegills further destabilizing the quadrant. Once things are taken care of Sela shows up again and you travel to another Dyson sphere in the Andromeda galaxy which leads to the invasion of the Iconians who wage war on the entire galaxy. The only hope is by following clues and finding the Krenim, who hid themselves from the time stream. You convince them to join the effort and rebuild Annorax’s ship from Year of Hell. In one botched attempt, the lead researcher Noye’s wife was removed from the time stream along with most of her species. In a final act of desperation you get sent back in time to the fall of iconians to gather Intel and find a way to stop them, along with a Klingon general and of course Sela. You find they’re a mostly benevolent peace people who use their version of the Prime Directive to not give “lesser” races access to their highly advanced tech. This pissed those races off enough that they all allied and bombed the iconians to near extinction. During this attack you’ve befriended the leader of the iconoans, along with the Klingon general, meanwhile Sela is the devil in your ear and just wants to kill them all. The iconoan leader asks for your help to escape with their collected knowledge, the World Heart, which you agree to do. While the 12 remaining iconians are attempting to escape via their gate…Sela shoots one of them and takes the World Heart and the iconians swear that she will burn for this as they leave. So yeah, that bitch Sela is the one who singlehandedly set all this in motion. You go back to your original time and return the world heart to the iconans and they agree to a ceasefire.