Carol Vogel says American collectors fear the tax man:

American collectors in particular are among this fall’s biggest sellers. Unsure of what the political future holds, they have decided to sell art now, fearing that capital gains taxes on fine art could rise from their current 28 percent, already higher than the 15 percent federal rate on the gains from selling stocks and bonds.

“That concern is what accounts for more discretionary selling this fall,” said Tobias Meyer, head of Sotheby’s contemporary art department worldwide. “Collectors are looking closely at their assets and evaluating their collections.”