Now that another reader has written in to comment on the apparent high squirrel mortality rate this season I've redoubled my efforts to get to the bottom of this, and I now believe that we may be seeing a recurrence of The Great Squirrel Migration of 1968. It seems like this rare occurrence is a lot like lemming migrations, except that the lemmings start moving because their current home is running out of food usually, while this does not appear to have been the case in past squirrel migrations. The article cites overabundant fleas and 'psychological factors' as the most likely causes. In 1968 they were swimming across lakes and would not be turned around, and were reported to be 'dancing in the road' by some observers. Still developing...
[Remainders: I've also discovered that highway mortality accounts for nearly 100% of male squirrel mortality, this intriguing tidbit that was not explained any further: "High food availability with low squirrel numbers, harvest intensity may be too intensive", and this adage: "Squirrels gathering nuts in a flurry, will cause snow to gather in a hurry".]