Can anyone tell me why my writing always runs? I usually finish cakes in the evening, and by the next morning the icing has run... Could it be the change in temperature...say from the refrigerator to room temp.??

I have never had my writing run but again, I do not refrigerate my cakes. My state licensing does not allow for icings or fillings that need refrigeration. So, my guess is it is from the condensation upon coming to room temperature as you have surmised.