Pumpkin Whoopie Pies

So I made these pumpkin whoopie pies a few weeks ago and I was totally amazed at how easy they were.  They turned out to be such cute little cakey cookies with a delicious cream cheese frosting.  But then, the weirdest thing happened.  I put them in an airtight container overnight and when I opened the container to serve them the next evening, they were watery.  As in there was liquid all over the bottom of the container.  I have no idea what happened.  Any ideas??  Is that a pumpkin thing?