Sometimes things happen which I just don't get. First of all, what would make a senior male federal agent think that giving a female AUSA a noogie was OK? Second, did there really need to be a year long investigation into whether it "was foolish horseplay, bullying, or harassment." The answer to all three is "Yes." Has no one in the FBI worked with men before? Do they not realize that this is part of a typical male acceptance/testing/alpha dog process?
What was the appropriate response here?1 Well, she could have just laughed it off. She could have elbowed him in the ribs (or other sensitive areas). She could have pulled a practical joke on him the next time he came to meet with her. All are fairly typical bonding ritual responses.
Why'd it go further? Hard to say; going forward on something like this doesn't really help anyone. She comes across looking like she can't work well with others; he comes out looking like a neanderthal who never heard of the initials P followed by C. She gets what sounds very much like a demotion, from the Organized Crime Strike Force to "handles drug cases." He gets fired (although this seems as much because he lied about the incident).
1 I am, of course, assuming the typical 4 to 6 swipe noogie - not a 10 minute rub 'em bald version.