I had something similar happen a couple of weeks ago. I chalked it up to not knowing what I was doing since it was my first century and first organized first group ride. Like you I felt strong at first, in fact impatient with the group I was with on the rolling hills. About mile 60 after a stop I felt the beginnings of leg cramps but I spun them out which usually works for me, and subsequently was considerably weaker on the hills. In my case it was not drinking enough and not enough hill work at that kind of distance.