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 answe

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.