Here’s a terrible idea unassociated with any existing world, sadly.
Aspected mana catastrophe.
On this world, the mass death and disruption associated with colonisation has disrupted the old stability of the North American mana fields, tipping many of them into a self-perpetuating Gate aspect which spontaneously formed massive, largely-stable one-way gates into the distant past. Dinosaurs emerge, and rapidly reproduce, filling the ecological gap left by the human population collapse.
The dinos are a new challenge for the settlers. They can’t be turned against each other. They can’t be lured into deceptive treaties that are swiftly broken or blackmailed into compliance. They don’t have a culture to undermine or subvert. They don’t give a damn about smallpox, and they carry their own microbes. Plenty of them don’t give a damn about period firearms, either. Some of them are very, very clever. Some of them are vengeful and will lead a hundreds-strong herd to trample your frontier town into the ground because some fool killed their kin with an elephant gun. Some of them are happy to chow down on those nice big herds of cattle the settlers have and make ranching a very bad idea.
The Native Americans, used to coexisting with megafauna and varying degrees of nomadic existence, and knowing the landscape and ecosystem, handle this far better than the settlers.
The same phenomenon could create other, smaller, less predictable gates that do what you want and get attention from the I-Cops.
If the Cabal were involved, they might have been experimenting with mass mana aspecting for various reasons. The link was an accident; now they’re studying it to see what went wrong, and perhaps to exploit the phenomenon. Alternatively, the link has staved off settlement and they are buying time to learn everything they can from the Native Americans. Or it may not be a whole-Cabal thing at all - maybe a particular faction wants to stop the colonists out of actual benevolent motives (which might include being survivors of colonisation themselves). Maybe someone just really likes dinosaurs! (insert obligatory comic here)
Centrum may be experimenting with tipping the world into their own sphere of influence, as they’ve done before. It could also be a deliberate distraction designed to keep Homeline attention off something more important (and a good one, because time gates releasing dinosaurs will intrigue just about anyone, let’s be honest).