The author of the Daily Herald article seems to be surprisingly incurious as to motive.
Aren't they all?
The motive cannot be assigned until the voter registration records of the perpetrator have been obtained.
If he is a Democrat, the motive will be frustration and rage at having lost his job and concern over the shredding of the social safety net.
If he is a Republican, the motive will be incendiary rhetoric, particularly by Sarah Palin and/or the Tea Party, and general animus toward the government.