Youtube Video was playing mysteriously in background

This isn't a major concern but more of a curiosity. I had a Youtube Video playing in Firefox and when I went to share it on another application, it started playing again causing it to play twice. Force Quitting off all applications only stopped the original play of the video, but the “phantom” one continued to play until I restarted my Macbook. Again posting this as just a curiosity as to what would cause this, and what a better solution might be in the future.