If 100 Gbps of capacity suddenly went away, then BGP attempted to route around it, and sent that traffic down 10 Gbps routers instead, which overloaded, or were not ready for the deluge of traffic, then those BGP routes could start flapping, at which point those routes would get pulled, forcing it onto yet other circuits, which may be 1 Gbps and that's how the outage continues to grow.

Too much capacity is centralized in certain providers, and if they go down there simply is no backup route that can handle the same traffic patterns.