There are several things that can cause cracking, adding water to the mix partway through the pour will result in unequal curing & possible cracking. Over using a concrete vibrator at any one spot can separate aggregate from the mix, causing weak spots that are susceptible to cracking, & unless you altered the mix to allow for the temperature, using type II cement or adding a retarder, the concrete would have cured at an accelerated rate, which can cause cracking.

 Also, if the wall was not covered & kept moist for the first week, then the water would have evaporated prematurely, causing the concrete to dry & crack rather than cure. If the pattern of the cracks matches that of the rebar, then hot rebar could be the cause. That rebar at a temperature of 100 degrees is hot enough to drive the water from the concrete that is surrounding it, causing a differential cure rate & cracks.

Fortunately, hairline cracks are more cosmetic than structural. Seal the cracks with a masonry caulk that will prevent water intrusion & damage than can be caused by freeze/thaw cycles & monitor to make sure the cracks do not get bigger.