There's also how the immense amount of aether in play from Louisoix attempting to (sort of) summon the Twelve caused both healing and shifting (a bit) of other areas within Eorzea - lot of stuff going on with the aether of the land during that time.
For Coerthas, it's the fantasy version of what happened in The Day After Tomorrow, but instead of hurricane-like blizzards pulling in "super cooled" air from high in the atmosphere, it's a shifting of aether bringing in ice aspected aether that leads to a similar sort of snap freeze...though not quite as immediate (getting frozen where you stood isn't described as happening, but getting caught in rising snows or water that freezes suddenly was apparently a real threat).
