Not necessarily anarchy, a government move to systematically eliminate Christians and conservatives by violent force would be a SHTF moment too.
It basically means a situation where the old rules no longer apply.
It’s really hard to tell if internal or external attacks will cause the attacks, likely a combination of both. For instance, if we had nuclear attacks on the U.S., which could happen given all the good stuff America is doing at drawing down its military and pissing off the enemy, well a nuclear attack would cause anarchy, since communication with the federal government is dead, and most of the population is too.