I'm not at all surprised that early voting would be "Democrat heavy". I think it would be the case even if Republicans were slightly ahead in the opinion polls.

There are many more incentives for urban voters to vote early than there are for their suburban and rural counterparts. It's the urban precincts that are most likely to have long lines and other SNAFUs on election day, and urban voters tend to be Democrats.

Early voting is not as attractive for many suburban and rural voters, who tend to be Republicans. Why would anyone want to drive 25 miles to vote early when the local polling place is just around the corner and isn't really all that crowded on election day?