A shocking allegation has rocked Hollywood — Kevin Federline’s upcoming memoir “You Thought You Knew” accuses Britney Spears of disturbing late-night behavior with their sons. The bombshell knife claims allege that Britney’s children once woke up to find her standing silently in their doorway, holding a knife. Federline says he fears for the pop star’s well-being and warns her life is “racing toward something irreversible.”