What is the reason for setting flaps 1 on the ground at high temperatures?
From the A320 Flight Crew Operating Manual (FCOM):



And the image to text:


  On ground, hot weather conditions may cause overheating to be detected around the bleed ducts in the wings, resulting in “AIR L (R) WING LEAK” warnings. Such warnings may be avoided during transit by keeping the slats in Configuration 1 when the OAT is above 30 °C.


(Emphasis mine)

Basically it lets the leading edge anti-ice [hot air] leak sensors breathe, and not display a false positive.

Note that moving the lever to 1 from 0 or 2 at slow speeds (on ground) is called config 1+F, which deploys the flaps as well as the slats (see below).