AT&T wasn’t the only one... Verizon, T-Mobile - around a dozen smaller carriers all had issues.
And, it wasn’t a solar flare (didn’t affect internet, same electric power sources, same satellites; and the bulk of the recent big flare centered over Africa - NOT the US).
Because they were contacting AT&T customers.
I am not sure the others really had any issues beyond the normal everyday individualized outages that happen everyday. The media used downdetector.com when the reports started and they saw tens of thousand outages for AT&T and a few hundred for the others and went with that reporting. Note that downdetector is crowdsourced and relies on individuals reporting outages so it is possible that people that were having issues with AT&T reported on the wrong company.
Don’t trust the media.
It must have been geographically determined. In my area Verizon and T mobile were fine. The problems came from AT & T users attempting to communicate with other carriers and vice-versa.