The real story is that Stadia employees had no clue, and were operating normally until the announcement. (Even if employees don't know 100%, typically management will put a freeze on some operations with a vague excuse)
I disagree, a slight thing but worthwhile disagree, I think useful to remaining an integrated and active human: the premise that there is a "should not" about being stunned presumes feeling surprise is somehow bad, a "should not." It's okay to be stunned.
Feelings can be inconvenient and wrong / only part of the picture. And you I assume want a deterministic life. SO, learn to cope with feelings.
Generally, accept feelings and reactions as a part of existence. Otherwise you (human) will start to ignore your feelings and get neurotic.