Yahoo"s Bradley Horowitz, head of an internal start-up unit at the company, has left for larger rival Google. Horowitz, vice president in the advanced development division, called the timing "a bit unfortunate" on his personal blog Thursday, and said his move isn"t related to Microsoft"s bid to buy Sunnyvale-based Yahoo. His departure comes two days after Salim Ismail, who led a related Yahoo unit called Brickhouse, said he was leaving. Horowitz, who joined Yahoo in 2004 to work on its search engine for finding audio and video clips, oversaw efforts to accelerate the pace of product development. He held events such as Yahoo"s "hack days" for developers and started a blog called Next to highlight new products.