Investigators are trying to figure out why a six-year-old was crossing a busy highway late Friday night.Kallie Palmer wandered into freeway traffic on interstate 35 west in Burnsville, where she was struck and killed by an SUV.The girl had been playing at a friend's house, but investigators are not clear what happened after that.Police say the home's yard is encircled by a chain-link fence that would be about waist-high on an adult... Palmer may have climbed over the fence.