In Act I, Scene 3 of "Julius Caesar," why does Casca have his sword drawn?

In "Julius Caesar" on the night before the Ides of March, a frightened Casca, with sword drawn against the danger he fears, encounters on a street in Rome Cicero, who asks Casca "why are you breathless?  And why stare you so?" (I,iii,3).  Casca tells Cicero that he has seen a common slave pass by, holding up his left hand from which "twenty torches joined"(I,iii,17) burn without harming the slave's hand.  Also, Casca says that he had just put up his sword near the Capitol when he met a lion that simply stared at him and then passed on.  In addition there were men "all in fire" (I, iii,25) who walked up and down the streets, and owls that were out at noon on the streets.

All these bizarre sights Casca interprets as portents of evil to come, believing that the gods are either engaged in civil war, or they are determined to destroy Rome.  When Cassius enters the scene, Casca reports these incidents to him.  His sword having been drawn earlier may symbol Casca's bloodlust and the future murder of Caesar since he informs Cassius that the senators plan to make Caesar king.  To this report, Cassius replies that he would rather kill himself than see Caesar king; furthermore, he tells Casca of a plot to assasinate Caesar and convinces Casca to join the conspiracy:

I know where I will wear this dagger then;/Cassius from bondage will deliver Cassius....That part of tyranny that I do bear/I can shake off at pleasure. (I,iii,89-99)

Readily, Casca replies,

"So can I;/So every bondman in his own hand bears/The power to cancel his captivity. (I,iii,100-103)

That Casca is a bit devious and is rather influential with others is also evidenced in his conversation with Cassius who calls him "dull" (I,ii,57), while, interestingly, shortly before this statement, Cassius has remarked to Brutus that Casca feigns dullness--

However he puts on this tardy form./This rudeness is a sauce to his good wit,/Which gives men stomach to disgest his words/With better appetite--(I,ii,299-302)

--in order for people to better digest what he says and to feel that they have come up on their own with the ideas that Casca surreptitiously suggests.