Fun fact: Both are located in Norwalk, CT., about a 5-10 minute ride from each other. Priceline is located almost on the Darien border and Kayak is nestled in South Norwalk (SoNo). I gotta believe this proximity lead to board member and executive coziness.
To save you from reading that document, a law firm has decided to 'investigate' whether Kayak's board "breached their fiduciary duties to stockholders by failing to adequately shop the Company before entering into this transaction and whether priceline.com Incorporated is underpaying for Kayak shares, thus unlawfully harming Kayak stockholders."
Breach of fiduciary duty lawsuits are very common for all major acquisitions. For the law firm it's high potential reward with relatively little downside. The mere fact of one or two being filed means pretty much nothing.