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So, if it was known in previous semesters that a test bank was used, why such a large statistical aberration this time around? It seems implausible that 1/3rd of the class just happened to memorise the answers accurately, and indeed, the distribution suggests that another factor was in play this time around.

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"So, if it was known in previous semesters that a test bank was used, why such a large statistical aberration this time around?"

What evidence do we have for the large aberration?

Are there older reviews of the professor available online that say his tests are easy or that he reuses old test questions?

He showed slides of a normal distribution for the previous semester's results, vs a bimodal distribution for the current semester - that's the aberration I'm talking about.

The first explanation for that that pops in to my mind is someone googling/searching/researching a question they saw in the exam that they didn't know the answer to and finding it as a part of a test bank, then realising all the other questions in the test bank were on the exam too. He tells someone, they tell someone and next semester 200 students have a copy of that test bank.

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