ProcessUpdate and Partition Scans

I'm working on minimizing the cube processing for a cube with some two billion rows in a fact table. I put together an ETL package that processes the dimensions with ProcessUpdate following by processing the latest partitions. I've noticed that processing one of the dimensions , which happens to be the largest (some 1 million rows) and most complicated dimension, resulted in partitions scans. The SQL Profiler showed the scans with the following events: