I'll take a chance here and say this was a hack, possibly at the equipment-level. One major carrier having an outage? Possible. But three? On different networks? Even if its not a hack I'd love to see the root cause on this one! Communications is critical infrastructure, so I'm gonna guess the government will demand a full report.
When you lose your own network, your phone connects to a different one to be able to make emergency calls. One carrier knocking down other carriers is entirely plausible.