To be honest I didn't until the early twenty-teens. I started writing and the term was dropped in writing groups. In context the meaning was so obvious that it needed to explaining.
A plot bunny is a story idea that is not fully formed but that has a life of its own. It won't leave the author alone until it gets some attention. Sometimes they come as songs. One of these bunny's attacked me the other day and by the time I was done one brother loved it, one sister liked it, and one brother glared at me and coldly ordered me to burn it with fire...