"I was drinking plenty of water, and had plenty of gel."

The "plenty" part is a subjective judgement. What's plenty for you may be completely different from someone else's definition. What was the EXACT amount of water you drank? And during what time-frame?

I've done a lot of 100+ degrees-F rides in Vegas. The dry desert heat really dehydrates you quickly. I've found that I definitely needed salt-tabs along with my energy drinks. Also went through 3 full 20-oz bottles of water per hour! So for your 85-mile ride, I'm guessing you'd need at least 15 20-oz bottles of water. By your 65-mile mark, you should have consumed at least 12 20-oz bottles. If you had less, then you were definitely not drinking enough. Also the cessation of sweating is a sure sign of impending heat-exhaustion. Continuing after that will lead to heat-stroke which can be lethal.

Don't wait until you're thirsty, drink lots and early.