Did any other profs see a similar trend? Did the forms change or anything else about the review process? Were the ratings from comments or from the multiple choice exclusively?
My feedback for classes was normally very good, but two things affected it oddly. Wearing dark clothes lowered my scores. Also Auditors graded lower.
This stuff comes and goes; if you're seriously worried, there are usually programs on campus which will send an observer or two by to give you pointers from people who aren't lazy-ass undergraduates.