At a law forum I asked admissions officer Amy Stevens of USC how USC determines sections. She said they manually select people for sections in order to create diverse learning environments. I don't know exactly what they use (ethnicity, state, etc.) but it's not random.
I asked her if this is standard, and she said no.
Section stacking sounds terrible! I'll make sure to ask this question to schools that offered me a scholarship before I enroll. Thanks for the tip.