Im wondering what causes fondant wrinkles. After covering the cake in fondant sometimes i womt finish decorating the cake until the next day. Sometimes the fondant starts to wrinkle on the sides and on the top. What causes this? 

Some people think cakes settle and tell you to weigh down the layers w/a heavy book or such before applying the fondant.  Personally I have never had that problem and therefore never have weighed any of my cakes.  Could it be your fondant is too thick? 

Are you leaving your cakes at room temperature? I have often had them start to soften and sag at room temp, so I keep them refrigerated (even covered with fondant) until time for delivery.

I'm sure there is a vast difference in places that are hot & humid but I have done 99% of my decorating in HOT, dry CA  and never put cakes in the frig. I don't remember ever having them sag - especially to the point of wrinkles.  I have had a few 'blow outs'.  I didn't get into using fondant until the last maybe 10 yrs (of my 30+ yr career).