if your getting this info from SAM i'm not sure how valid it is.
It is to my understanding that the 2001,2002,2003 UAI's calculated on SAM are done using the same scaled marks. That is, the marks aren't re-scaled to their appropriate years.
I think it is arguable whether this is a correct way to do it since you cannot compare UAI's over different years as the candidature is different.
In conclusion it is only an illusion that the UAI is "dropping"