At low speed there is less ram air, and greater reliance on the fan/rad. I had a blocked rad, which caused low speed overheats, dumped coolant periodically too. As soon as I changed the rad, problem solved, but not before i'd needlessly replaced the t stat and viscous fanYes it was low speed. Probably 25mph up a slight hill (a lot of 20mph zones around me).
It was also only about half a mile from a 5 minute stop where I picked up my kid form nursery, so the engine shouldn't have been that hot to start with (unless it overheated on the way to nursery and I didn't notice, I don't think this is the case however)