Well, you have asked an interesting question because when Betty does start to scream it is highly revealing to watch how different characters interpret her outburst. Note what we are told in the stage directions:
The words "going up to Jesus" are heard in the psalm, and Betty claps her ears suddenly and whines loudly.
ABIGAIL: Betty? She hurries to Betty, who is now sitting up and screaming. Proctor goes to Betty as Abigail is trying to pull her hands down, calling "Betty!"
Of course, Mrs. Putnam is eager to seize on the fact that it was the psalm - the words of Holy Scripture - that started Betty screaming, interpreting it as another sign of devilry and witchcraft. However, Rebecca has a completely different interpretation. With the gentleness that "exudes" from her, she is able to calm Betty. Note what she says about how she managed to calm her and what she thinks is the problem:
I think she'll wake in time. Pray calm yourselves. I have eleven children, and I am twenty-six times a grandma, and I have seen them all through their silly seasons, and when it comes on them they will run the Devil bowlegged keeping up with their mischief. I think she'll wake when she tired of it. A child's spirit is like a child, you can never catch it by running after it; you must stand still, and, for love, it will soon itself come back.
Crucially, we have two very different interpretations here, which of course leads on to the rest of the action in the film. People are too ready to side with Mrs. Putnam which leads to the large numbers of people who are executed and who come to believe in witchcraft for themselves, however, the position of Rebecca offers us an alternative approach which is more rational, based on experience and love. Of course, Miller is really asking us which camp we side with...