Whenever one of our opportunity stages is changed to one of two custom closed/won states, our closed date is automatically updated to today's date.

While I can understand this is behaviour some users might actually desire, this is NOT behavior we want.  Further, this is new behavior - it didn't used to happen.

I have checked field validation rules (we don't have any for opportunities), and anywhere I can think of in the custom settings.  I can't find a culprit.  Can anyone point me in the right direction to disable this behavior?

Thanks!