It is essentially a matter of geometry and optics. In an ideal judging world, each model would be placed on an individual stand perfectly lit by several banks of dedicated lights -- where the judges could walk completely around it, view it up close, view it from afar, view it from above, and view it from below. Would require a venue about the size of Utah and a budget about twice that of the US's..
But with multiple models on tables which allow only one side from which a judge can view (sometimes two), covered with models, some of which are only inches from each other, some of which are teetering on the edge, some of which are at the absolute back of the table, some of which are lighted and some of which are in the darkness -- it becomes absolutely necessary to move models.
Add to that the common practice, not yet mentioned, by contestants who enter their entries later or who decide that their chances for recognition are improved by optimal placement -- moving other entries already on the table. Yes, I said, "...common...". Cause I've seen it hundreds of times.
Models get moved. Some out of necessity, some out of convenience, some of out competitiveness. With the introduction of reality, we do the best we can.