I'd venture a guess that it started out with someone offering the service of providing 911 access, and it worked fine at small scale. Then they found more customers to connect, because traffic was very low for the infrastructure they already had. Then more and more jumped on the bandwagon, to offload their "911 responsibility" to this one popular provider. Adding more users was practically "plug-and-play", they just had to adjust some routing tables and bam, their 911 responsibility had been outsourced and now they can forget about that annoying and expensive thing the government mandates them to do "because we have this other outfit taking care of all that for us".
Then over time this one scrappy little outfit found itself the sole 911 service provider in three states. But instead of using all that new capital from all their new customers to upgrade their now over-extended systems and add redundancy, they just pocketed most of it. Their customers were happy, their shareholders were happy, life was good. Nobody thought there was anything wrong because there hadn't been any problems, and the 911 calls were getting through. There were no reliability or single-point-of-failure audits because there had never been any problems with it in the past.
Then a wild posthole digger appears, and suddenly everyone is vividly aware of just how fragile of a system everyone in the area is relying on. And the finger-pointing begins. "We never could have imagined something like this could possibly happen!" Oh yeah, ignorance is bliss.
I wonder if circuits like this are being overused due to low prices? Like if you have a "weak link" circuit that is one of the few that connects popular points X and Y together, and "in theory" there's a lot of users for that circuit, but the smart ones realize they can't put all their eggs in one basket through there (like their primary AND their backup system, you can't run BOTH through the same node, for fault-tolerance reasons) and so the owners of that circuit find they're having trouble selling capacity through it, so they lower prices. This attracts outsourcers and resellers to reroute through there for the low prices. Then some groups, looking to cut costs, find this new outfit offering bandwidth from X to Y and switch to it, without realizing that now their backup service is running through a node that their primary service does. I'm sure there's processes in place to catch when this might happen, but I wonder if the lower prices (due to those 'aware of the danger' avoiding the node) is what's leading to unexpected loss of redundancy for the less diligent groups?