Article continues below advertisementKevin Federline continues to drop bombshells about Britney Spears' concerning past behavior.In an excerpt from his new memoir, You Thought You Knew, the pop star's ex-husband, 47, revealed what led to her shocking shaved-head incident in 2007.Article continues below advertisementSource: @britneyspears/InstagramBritney Spears frequently puts on erratic dance displays on Instagram.At the time, Spears, now 43, had checked herself out of a rehab facility and "showed up at the gate, ringing the bell and shouting a bunch of things that didn't make sense.