Good info cyccommute, thanks. I was hydrating very well and had electrolytes, potassium pills, B vitamins and a quality eGel. In the end, it wasn't enough. So beside aasking here, I did some good ol' interent research and found this

How to Prevent Leg Cramps When Cycling | Tips for Preventing Cramp

I think what I was experiencing yesterday was neuromuscular fatigue. For sure, next long ride on a warm day, I will be better prepared and expect better results.