I know the other carriers are saying they aren't affected, but one look at DownDetector shows that nearly every carrier was affected, and all at the same time.
A user who has a working cell phone could contact a user who is on a network experiencing issues. Because the call fails, that user may decide their cell network is also faulty. Downdetector only works on user reports. Its basically useless for actual measurement because people are bad at troubleshooting.
That site counts mentions over social media, if you said 'T-Mobile/Verizon isn't having an outage' it'd still show up as outage activity on it. Plus people report issues calling AT&T customers.
There was an earth directed X class, but not a big one. If that could affect SS7 I would expect it to also take out a chunk of the internet but I am not seeing that. [1] There are many cellular networks having issues but some could be and likely are resellers of others. [2] Probably more likely SS7 related as marcus0x62 mentioned. Another potentially SS7 related possibility? [3]
Could be a hack Could be a single point of failure Could be a config change that borked the system
Could be other things that make more sense we will have to wait for more info.