The vast majority of cases were "simple cheating" (e.g. sharing answers during a test, collaborating when collaboration was specifically forbidden). Of these, a large percent were foreign exchange students, who explained the behavior as either a difference in the nature of education (individualist in the US vs collectivist in their home country) or a claimed misunderstanding of the instructions due to language barriers (despite reasonable TOEFL scores required to be admitted).
From talking with faculty who had served on the committee for longer than me, while those were occasionally the issue, more often than not students cheated for one of two reasons: 1) external pressure for them to get a good grade (e.g. parents paid tuition proportionally to GPA, or punished for poor grades), or 2) desire for credential over knowledge or apathy towards the education itself.