I did happen very, very quickly, within a day or two as you said.
It's a huge decision to make, knowing that it will likely be for a very long time. His father had a business to run, his mother had a job to go to, and I'm guessing the daughters had either work, school or a combination of them both. They all would have had friends, commitments, potentially partners for the girls etc. Anyone who's moved an entire family knows it doesn't just happen overnight, in ideal circumstances anyway. Even a last minute weekend away or holiday takes some planning. However, if it was all very last minute, I guess you grab all the bags you can, fill them with your immediate needs and wants, and hightail it out of there, worrying about all the finer details later. Perhaps have a close friend access the home later to take care of things that you couldn't do last minute. It's hard to know what the long term plan was for them - leave until the immediate circus died down, leave until the trial is over, leave for several years until it's no longer such a hot topic, or leave for good. The area they lived in would still have a small town feel in these circumstances, with gossip, stares, crazies camped outside your house etc.
It leads me to wonder if someone within his immediate family knew or suspected something, and could have informed police of their concerns? Perhaps they had somewhat of a "heads up" that the you-know-what would hit the fan in the next day or so? This option would give them a bit of thinking time before news broke and the decision to leave wouldn't be quite as last minute as it appeared. You also have to wonder if LE advised them to leave, for whatever reason. Retribution, if indeed someone within his family dobbed him in? All my thoughts only.