I don’t doubt that high temperature and lack of water were to blame in your case. But vigorous physical activity even when well hydrated and in modest temperatures can cause very painful leg cramps especially if you’re old [like me]. I have no ill effects during 99% of my (mostly single-summit) SOTA walks [sometimes my legs are a bit tired the next day] but in recent years on two arduous all-day walks I got agonizing thigh cramps which forced me to rest for 10-15 minutes.
One was on the descent from Scafell Pike G/LD-001 and Scafell (not a SOTA) after 5 hours of steep walking and scrambling (6½ hours in total).
And the other was during the walk from Brodick ferry to Goat Fell (GM/SI-006) and back. I was jogging [not walking] in hiking boots, with big rucksack and dog on a lead down the path trying to make the ferry before it left [I could see it sailing to Brodick during my descent]. When the thigh cramp hit, I had no choice but to writhe in pain then sit on the ground until the muscles relaxed. Worst of all I had to skip the customary fish supper in Brodick to get to the ferry gates just in time before they closed them. Madness!
Things we do for SOTA.