I think the explanation for her using it is much simpler: she's deliberately not completing the sentence, in her mouth, or in her head.
The full sentence is: "everything happens for a reason, and in this case it is my stupidity".
She's just expressing the "I don't want to think about my actions and consequences" in a different, more subtly avoiding manner. As long as she doesn't attempt to think through to an actual conclusion, she can pretend the conclusion is beyond comprehension or just stemming from the natural order of things, instead of just being a few minutes of honest thought away, and ascribable to personal individual action, and thereby sustain safely undisturbed her fundamentally lazy form of living.
The full sentence is: "everything happens for a reason, and in this case it is my stupidity".
She's just expressing the "I don't want to think about my actions and consequences" in a different, more subtly avoiding manner. As long as she doesn't attempt to think through to an actual conclusion, she can pretend the conclusion is beyond comprehension or just stemming from the natural order of things, instead of just being a few minutes of honest thought away, and ascribable to personal individual action, and thereby sustain safely undisturbed her fundamentally lazy form of living.