My understanding is that judges have certain specialties - one judge might be well versed in a particular area of law but not other ones. The case I was on was an area where nobody in the district had expertise, and everybody (judge, prosecutor, defense, jury) was learning as they go. The new incoming case was one that was in an area that our previous judge would normally handle. So it was assigned to him because it came in through his department, while the case I was on was sort of a free-floating orphan where not much was lost by having another judge handle it (and it was also already in the jury instructions phase, with testimony complete).