Final answer:
Cassius is surprised by Popilius's message because it suggests he might know about the assassination plot against Caesar, threatening the secrecy and success of their plan.
Step-by-step explanation:
Cassius is surprised by what Popilius Lena says to him as they enter the Senate House because the content of the message unexpectedly indicates that Popilius might be aware of the impending assassination plot against Caesar. During the tense moments before the act, any suggestion that their plan has been discovered would be a shock, potentially implying that their conspiracy is no longer a secret, which could threaten their success and lives.