Just a quick Google pretty much comes back with a bunch of nuclear protocols.
EAM - nuclear protocol
Movement of Pelosi/Pence would be for chain of command
Castle_Rock - Castle under[score] rock?
Castle is a term for a nuclear fallout or nuclear test site
Maybe there's evidence somebody may try to hit the US with an EMP?
Lots of possible meanings here, but I think we'd be hearing something from POTUS if somebody wanted to skilled-fry us with a nookular.
Implying he may be in some legitimate danger though?
DEFCON scales?