From my limited observations there seems to be a culture of work sharing among foreign students. Some of it can probably be put down to lesser English language skills, they feel a lot safer following anothers lead rather than attempting to write something themselves from scratch.
It took me an absolute age to fix; at one point the tutor basically admitted I must be telling the truth and "offered" me half the papers marks.... bah.
Also, I did an Engineering degree; lots of plagiarism going on there (most engineers are pretty bad authors it seems :P). One major problem they had was the huge section of Asian students who all hung out together (across years) and basically shared/co-wrote all their work... I know one lecturer rejected about 35% of the drafts in one course during the second year!