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A show I saw on HGTV mentioned this kind of behavior. It's to make them look like one large organism when they all do that behavior. To keep birdies from thinking they are big enough to eat that 'huge' animal.
This behaviour is common among sawfly larvae. And my first thought was that these were sawflies. It was by searching for Sawfly and Oak on the net that I found a site on caterpillars that feed on trees and shrubs, http://www.ces.ncsu.edu/depts/ent/notes/O&T/shrubs/note07/note07.html which includes several sawflies as well as caterpillars like these. Checking further the species that mentioned oak led me to your caterpillars.