There are a lot of people who would say that the volcano in Fallen Kingdom was just put there in the plot to add conflict for the film, given how suddenly the warning comes. Many volcanos in real life (especially Stratovolcanos like the one we see in the film) can go tens of thousands, if not millions of years between periods of activity; yet Mount Sibo erupts only 3 years after the fall of Jurassic World, and 25 years after the original Jurassic Park. Both of those are extremely short on a geologic timescale, and make both dinosaur escape incidents on Isla Nublar utterly pointless if Mount Sibo could have erupted at any time when those parks were or would have been open to the public.
However, I don't think that's the case at all, and that the eruption of Mount Sibo happened at the time it did for a very specific, and man-made reason. But before I can explain why, we must address the history of Mount Sibo's volcanic activity.
The volcano had been active and pumping out clouds if steam since Isla Nublar's discovery in 1525, thus giving the island its name "Cloud Island". InGen had bought the island centuries later and developed it into the site of Jurassic Park; building a geothermal energy plant that would harness the power of Mount Sibo's heat to generate electricity for the theme park and all of its systems.
We would later see the geothermal plant in the 2011 video game, Jurassic Park: The Game, where one of the missions involves the player releasing the steam pressure to prevent what Dr. Laura Sorkin described as half the island exploding. While that last part was most likely Laura just exaggerating, it sounds similar to what we see happen in Fallen Kingdom, except for the massive amount of flowing lava.
This leads us back to Mount Sibo's 2018 eruption, which was reported on by the Dinosaur Protection Group website a few months before the release of the film. Some of the sites other articles report on details that are partly inspired by the events of the aformentioned 2011 game, such as the dead Herrerasaurs, and Jerry Harding's statement that Denis Nedry's sabotage of the park was done to smuggle embryos to a rival genetics company. Jurassic Park: The Game's relevence to the canon of the films is even further made aparent by the website, with the naming of Mount Sibo itself, first named in the video game. It's possible that due to this hightened relevence, there are some more dots that could be connected between the video game and films.
So going back to the geothermal plant, I believe that it's actually the reason why Mount Sibo erupts by the time of Fallen Kingdom. With no humans to maintain the plant again, it most likey built up pressure over the 3 years since the abandoning of Jurassic World. Or maybe the volcano was always supposed to erupt, but the power plant tapping into the steam pressure kept it stable up to that point. Either way, that pressure had to be released somehow, so it must have been diverted back into the magma chamber and ejected out through the volcano cone.
To summarize, this may be just be considered a "fan theory", but there is evidence to suggest that the state of the geothermal energy plant seen in Jurassic Park: The Game has some influence over the volcanic activity of Mount Sibo. This would also serve as an explanation as to the timing of the eruption event seen in Fallen Kingdom, giving it a reason why it happened so soon in relation to the demise of the two theme parks.