Frosting Won't Stay on Cupcakes?

I found a homemade buttercream that is to die for. It is easy as heck to make and it is fabulous for piping, plus completely delicious. It's made with confectioners sugar, real butter, vanilla, and a little heavy whipping cream. I made cupcakes yesterday and piped a beautiful swirl on top (cupcakes were completely cool), then put them in an airtight container (in my van where it was cool, like 45 degrees) so they wouldn't dry out and I wouldn't forget them. It pipes out beautifully, but today when I took it to work, a coworker grabbed one and tipped it a little when eating it and the whole wad of frosting just popped right off the top in one magnificent swirl. Not sure why this would happen. Any ideas?