What causes an orange peel effect when spray painting?

Most likely cause is wrong reducer for the temperature. If you use a fast drying reducer in warm/hot weather, the paint will dry before it has a chance to level out. There are 3 basic kinds of reducer for enamels, Cold, Mid temp, and hot weather. You don't mention what type paint you are using, so, I am pretty much guessing what it is. However, this rule holds true for most all kinds of paint used today, base/clear, acrylic enamel, and single stage urethane. Air pressure and paint gun adjustment also plays a role, but reducer is my guess.