The very nature of the opening scene goes against so many of Salem's tenets that it makes sense that there would be such intensity of emotions. The fact that Betty does not awake and no one can explain why is a part of this. The fact that Parris can provide no guidance to those who look to him for it helps to add to this. The insinuations and rumors of what happened in the woods only furthers this. In such an opening, Miller posits his own thoughts with his stage directions, reflecting a town that is constructed on the need to be certain. This absolute faith in certainty does not understand how to appropriate the idea of doubt and insecurity. When these elements appear in Salem, Miller suggests that Salemites lack the needed skills to discuss these elements in terms that make sense. Instead, they are prone to become worried and intensely high strung about the presence of such elements, taking it as an affront to their own values. This inability that Miller suggests through his stage directions is embedded in the town's foundation is not only the reason why there is such insecurity and a sense of "stirred up" emotional contagion, but also why the Witch Trials took such a hold in the town.
To understand Salem's response to Betty's illness in the opening of Act I of The Crucible, you need to have a little bit of background on Puritan culture. For starters, Puritans were very self-conscious about their religious experiment in the New World. Like William Bradford said, the Puritans considered themselves to be a "city on a hill," with all the eyes of the Old World upon them. Any sign or symptom of the "devil's work" would have directly contradicted what they saw to be a very important moral mission. Additionally, this was prior to the Enlightenment in Europe and the Age of Reason in the United States, so any unusual occurence (what we would explain away, in this case, using child psychology) was considered to be controlled by divine workings. The Puritans also considered the Bible to be the literal word of God, and since the Bible talks about witches and demon possesssions, they would have been primed to respond to irregular behavior in the way that they do when The Crucible opens. Another potential cause is the severe repression that occured within Puritan culture. The possible bewitching of a young girl, the minister's daughter no less, would have been scintillating news. If you ever doubted our distant relation as a country to the Puritans, just compare the way we still react to the slightest whiff of a scandal, and you'll have a pretty good idea of how things got out of hand so quickly in Salem in 1692.