Due to "rolling frequency outtage" [sic]: From a claimed photo of FAA documents (I replaced the all caps with lowercase):

"... ground stop is in effect for all destinations until further notice due to rolling frequency outtage in multiple centers..."

and

"... the ground stop is now a nationwide ground stop for all centers, all destinations until further notice due to widespread frequency outtages.. ..."

https://x.com/Anna_Giaritelli/status/2121344915091206311

Which is cited in the Hindustan Times (leading Indian newspaper), which refers to the Twitter source as a journalist.

https://www.hindustantimes.com/world-news/us-news/faa-issues...