My back used to itch like crazy when I first started running. I think its from sweat, well not actually sweat but right before you actually sweat, espeacially if you body hasn't sweat much there before. Now I sweat like crazy and I havn't had a presweat itch in a long time.
mabe its like the body is trying to sweat but the sweat glands are all gunked up from lack of use and it causes an itching sensation. All I know is that was basically it for me. Itched alot untill I actually got a good sweat going, and now that I'm conditioned I no longer pre-sweat itch.