Why does Mayella Ewell break into tears at the beginning of her testimony?

Mayella's less than successful court appearance begins
when she breaks into tears early in her testimony.  She tells Judge Taylor she is afraid
of Atticus, and specifically is afraid that he will try to make her look like a liar
like he did her father.  There is a certain element of comic relief here, as Judge
Taylor scratches his head.  Scout observes that "It was plain that he had never been
confronted with a problem of this kind."  Further into her testimony, Scout makes
another observation about Mayella, that gives us a bit of insight into her character
and/or coping mechanisms:

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"Apparently Mayella's recital had given her
confidence, but it was not her father's brash kind:  there was something stealthy about
hers, like a steady eyed cat with a twitchy
tail."

At this point, the
reader may pause to wonder if the tears early in the monologue were real, or part of
Mayella's strategy to do well on the stand and/or avoid another beating by her
father.