My Honey had a very sensitive tail base. If you skritched her there, she would dance up and down...first alternating legs and then both at the same time. She loved to be skritched (honestly, I thought of it as foreplay, she loved it so much). I'm wondering if Sunny experiences the same sensations when she's super happy (nerve stimulation) and is scratching her own itch. Some dogs "hackle up" (including at the tail base) when they're playing or anticipating playing....if she does this, even if really subtle, the horripilation could easily be stimulating her nerves there.