70-90oz across ~3+ hours of riding in hot conditions, particularly if known to be a heavy sweater -- sounds like it wouldn't be sufficient fluids, to me.
Back in the day, I used to run distances quite hard, often in hot trail conditions, often for several hours. Proper hydration was critical, as with any other hard activity. Almost never did supplement (with electrolytes) on average hard runs, back then, even if a ~2hr route. But fluid intake was something else.
I used to do about 16oz every 15-20mins. Of course, if running, water weight gets tough to carry much (in packs/bottles), so stopping every once in awhile is vital. Assuming well-hydrated prior to a hard distance run, at 16oz every 15mins that'd be a half gallon (64oz) every hour. If a 2hr intense run, that could easily require 1gal of water for the 2hr stint. On a bike, I'd generally not push nearly as hard, but still.
My suggestion based on what you've described would be: either carry another 50% water with you on the bike (a larger Camelbak plus another 1-2 big bottles), else stop halfway in order to recharge your water stores; and add a suitable electrolytes mixture to your water. It's still a guess as to whether the cramps were due to fluids and/or electrolytes, but it sounds like it, given how it rapidly cleared up with the post-workout fluids.
I suspect that's likely to eliminate the cramping.