Does this happen every time? Humidity can be a big culprit when it comes to making cookies soften. What's it like outside, rain, snow, so humid you can't breathe? That happens here every now and then. Once, I was making dinner and icing cookies at the same time, not a good idea especially when you're boiling water for pasta! Like, how dumb is that?!
Were you using RI or Glace icing? Sometimes it can be beneficial to ice the cookies the day after they're baked. Hope someone else pops in with a more definitive answer.