Because eggs can be temperamental sometimes.
I can see my icing will fail when the stiff peaks starts to look like flaccid.
But do not fear.
First take a pot and fill it up to 1/3 full of water. Put it on the stove at medium heat and put a metal bowl ( or another pot ) over it, but the bottom should not touch the water. pour the icing mixture into the bowl, and beat ( a whisk is sufficient – It is to ensure that the heat is spread evenly ) the egg mixture will slowly but surely become hotter – but it should not become too hot that you are not able to touch it. After beating it over the heat for about 5-10 minutes ( the mixture should be hot enough to cause a burning sensation in your finger if you leave it if for a while ) .
Remove the hot mixture from the bowl and beat it with an electric mixer for up to 10 minutes or until it is in the wanted consistency.