I thought it was well established that the AIs rebelled against the Guardians and wiped them out. Maybe I'm on planet Cyberdyne...
The Guardians largely wiped themselves out in a huge civil war (their second one as an interstellar civilisation). The AI just finished them off.
Exact timings aren't 100% clear, but the war lasted over 100 years and it seems that the actions of the AI to finish it were very rapid, which puts that right at the end. Given that I'd argue that in all likelihood the environmental destruction had already been done by the Guardians, and may actually have been a contributing factor towards the AI decision to take the action they did.
Key logs are below:
History 18: This data describes details about the Guardians second civil war. The war was ostensibly fought between the people of the homeworld and the colonies, but in truth there were divisions even among individual city-states. The scale of the conflict far exceeded that of the first civil war. In human terms, the first Guardian civil war was little more than a political clash, albeit with some violent conflict, but the second was an all-out war between the abolitionists and the rest of society. The war eventually destroyed the Guardians’ habitats, forcing them to retreat into ‘sacred’ domes protected by powerful shields
History 19: This data describes details about the Guardians second civil war. The war raged for many years and eventually destroyed the Guardians’ habitats, forcing them to retreat into ‘sacred’ domes protected by powerful shields. Eventually even the domes were destroyed, or their shields failed, and ultimately the population died out. Their predilection for expending resources on honouring the dead – a key tenet of their religion – only served to accelerate their extinction. Records indicate some variation in the nature of this devastation, most obviously at sites consisting of multiple domes. In these locations it appears the inner domes were destroyed later than the outer ones, as indicated by the slightly lower radiation signatures in the inner areas. I can only surmise that the attackers returned, after an initial assault, to finish the job.
History 20: This data describes details about the Guardians second civil war. It appears that the Guardians approach to warfare vastly changed as the war continued. Warfare was conducted using automated battle fleets. At first these were controlled by the Guardians via brain implants, but later they became entirely autonomous. In the early stages of the war the Guardians fought each other directly, but within a decade – and after much loss of life – most of the fighting was conducted remotely. Soldiers were deployed only to occupy areas that had been captured by autonomous or remote vehicles, and eventually were not used at all.
History 21: This data describes details about the Guardians second civil war. The war raged for over a hundred years, and Guardian populations fell dramatically during this time. Fertility rates dropped due to increased radiation levels and because the Guardians considered it blasphemous to use technology to address a biological issue. Eventually, one of the competing forces triumphed, but by that point, all the Guardians other than those who had been exiled a century earlier were dying.
Language 21: As is always the case with research of this nature, finding answers tends to raise further questions. I’ve found references to another form of communication in this data packet, but not one used by the Guardians, or not all of them, at least. As far as I can tell it references some kind of foe or adversary. Combined with the devastation seen at a number of sites, it indicates the Guardians may not have been the cause of their own demise.
10/28 : Civil War Log – Annihilation This log concerns the end of the second civil war. Remarkably, it seems the artificial intelligences developed during the conflict became fully self aware at some point, and were horrified by the destruction unfolding around them. It’s difficult to get a sense of exactly what happened next, as the Guardians were not privy to the Constructs’ thoughts. But reading between the lines, I believe the Constructs determined that even if peace was restored the Guardians would never be able to transcend their violent natures. I believe they decided that the only way to preclude further violence – while giving their own burgeoning society the best possible chance of survival – was to destroy what remained of the Guardians’ civilisation. By this time, the Constructs had complete control of the Guardians’ munitions and automated war machines. Their attack, when it came, was swift and merciless. Strategic nuclear and chemical-weapon strikes were executed with a precision that only a machine race could accomplish. The few that survived were able to record what had happened, but they soon succumbed to radiation poisoning. The Guardians were utterly destroyed.