Selling by US corporate insiders in August surged to $6.1 billion, the highest amount since May 2008, according to TrimTabs Investment Research. The ratio of insider selling to insider buying hit 30.6, the highest level since TrimTabs began tracking the data in 2004.

TrimTabs' data on insider transactions is based on daily filings of Form 4, which corporate officers, directors, and major holders are required to file with the US Securities and Exchange Commission.

TrimTabs said insider activity is not the only sign the rally is about to end. The TrimTabs Demand Index, which tracks 18 fund flow and sentiment indicators, turned bearish for the first time since March.

For example, short interest on NYSE stocks fell by 10.3% in the second half of July and margin debt on all US listed stocks spiked 5.9% in July. Over half (51.6%) of advisors surveyed by Investors Intelligence are bullish, the highest level since December 2007.

TrimTabs also reported the actions of US public companies have been bearish. Over the last four months companies have been net sellers of $105.2 billion in shares.

TrimTabs Investment Research is an independent research service that publishes detailed daily coverage of US stock market liquidity including mutual fund flows and exchange traded fund flows.