Star Trek Online is suppose to be a continuation of the existing timeline (running under the time travel theory that changing history creates an alternate timeline, so the original timeline still exists, you just can't return to it from the past anymore). So it exists in a universe where Romulus is destroyed. I don't have all the details (I'm not able to play a lot because of the computer I have) but Romulus is suffering political turmoil. Factions are fighting for dominance, the Klingons are moving to annex star systems (and the reason they haven't gone full force is the war with the Federation) and Starfleet is trying to prevent Klingon expansion while maintaining stability even if the Romulans object.
Pretty much, there's essentially two factions that have arisen inside the Romulan Empire in STO: the remnants of the old empire led by Empress Sela, with backing from the Tal Shiar; then there's the Reunification movement and the Remans; led by the Romulan D'Tan and the Reman Obisek. However with Sela now missing and the head of the Tal Shiar, Hakeev, dead the Romulan Senate is once again in disarray as factions within it vie for power.
This leaves the Reunification movement, along with their Reman allies, unopposed to set up a new homeworld for themselves and to seek aid from both the Federation and the Klingon Empire.
Incidentally, before his death, Hakeev was in league with the returning Iconians; who were responsible for both Sela's disappearance and the destruction of the Hobus star, which ruptured subspace.