The proximate cause is an internal party dispute over MPs voting themselves a pay rise in violation of a party directive, which should have triggered Fiji's strict anti-party-hopping laws. But as a result of this dispute, the supervisor of elections finally noticed that the party constitution statutorily-required dispute resolution mechanisms. They were given a month to remedy this, but a mass-resignation of party officials prevented this, and as a result, the party has been dissolved. Its MPs will now be allowed to join new parties or become independents.
You may wonder how the party was registered at all if it had never had the required clauses in its constitution. The answer is simple: the regime's laws did not apply to the regime. The regime appointed the supervisor of elections, who ruthlessly enforced the law against the opposition (resulting at times in parties being kicked out of parliament). But it was never applied to FijiFirst, and clearly no-one ever bothered to even look at their constitution when the party was registered. That changed with FijiFirst's 2022 election loss, and the subsequent sacking of their election supervisor. Now there's some neutrality, and the regime's party is subject to the law like everybody else.
So now Bainimarama is in jail, his henchman Aiyaz Sayed-Khaiyum seems headed that way too, and his party has been dissolved. Hopefully the survivors of that party can build some new ones, and finally free Fiji of the legacy of the military regime.