What has Cassius accused Casca of before Casca expresses his fear of natural calamities? What mood is Casca in?
Cassius hasn't accused him of anything just yet: all he's done is told Casca that he's been running around in the middle of the storm - Cassius has
....bared my bosom to the thunder-stone
And when the cross blue lightning seem'd to open
The breast of heaven, I did present myself
Even in the aim and very flash of it.
Casca asks him why ("wherefore" - as in "wherefore art thou Romeo": "why are you Romeo?") he's done this - and then Cassius makes his accusation.
You are dull, Casca, and those sparks of life
That should be in a Roman you do want,
Or else you use not. You look pale and gaze
And put on fear and cast yourself in wonder,
To see the strange impatience of the heavens.
But if you would consider the true cause
Why all these fires...
Casca is genuinely shaken up and amazed by what he has seen during the storm: which, considering how dry-witted and vinegary he is in his earlier scene (Brutus even comments on how dry he's become: "he was quick mettle when he went to school") is one hell of a change. Which is, of course, a neat metaphor for what the conspiracy aims to bring about.