Translation: I have no idea what actually happened/happens, so I'll just make up some apologetic bullshit.
None of us here know.
However, given my career, I have been party to various Homeland Security/FBI briefings. Thats what they have said is a typical source of this sort of warning. Since the “chatter” is very rarely specific its rare that they can act on it, but they will release a warning with whatever helpful information they have.
Our particular interest was possible attacks on US infrastructure. This has been an ongoing thing over the last two decades.
The very first such briefing I had btw was by the FBI in a tech conference, It concerned the Unabomber attacks, in 1988.