Sirius XM stock has declined 20% over the past 12 months

Berkshire Hathaway purchased a small stake in Sirius XM, causing the stock to jump 9.1%.

The purchase was likely made by investment manager Ted Weschler, not Warren Buffett.

The new position is very small for Buffett, who typically makes purchases of at least $3 billion.

Sirius XM has a high short-interest ratio, which can lead to volatile stock movements.

The jump in Sirius stock is causing financial pain for arbitragers and other investors.