Why does my heavy cream go straight to butter?

A few nights ago I felt like eating some whipped cream and I didn't have any so I decided to make some with some heavy whipping cream I took the whipping cream a little sugar and some vanilla and beat it with a whisk for a while then when it started to thicken a little the whipping cream sepperated into a very thick cream which I found out was butter after I tasted it and some other very thin liquid I didn't try so why did the cream go straight to butter without turning to whipped cream