Why do the conspirators fear that Caesar may not go to the capital in Act 2 Scene 1?

In the opening scene of Act II of "Julius Caesar," Cassius is concerned that

The unaccustomed terror of this night,/And the persuasion of his augurers/May hold him from the Capitol today (II,i,199-201)

Cassius complains that Caesar has become superstitious of late; the seer has told him to "beware the Ides of March," and the augurers, officials who interpret omens have decided it is unfavorable for Caesar to undertake certain activities.  However, Decius says that he can "o'ersway him" and bend his feelings in the right direction. 

Deius proves true to his word, for in the next scene, despite Calpurnia's dreams of a lioness whelping in the street, graves yielding up their dead, warriors fighting and their blood drizzling upon the Capitol, all omens that danger will come to Caesar if he goes, Deius is able to convince Caesar that this day he shall wear a crown.  The dreams, he tells Caesar are good omens that this day "great Rome shall suck/Reviving blood (II,ii,92-93). Thus, the man of pride who dislikes flatterers is flattered into dying.