ONLINE was set as the rebuild option, but all orsome of the indexes are being done OFFLINE.

Minion Reindexing strives to run no matter what.  If you’ve got the ONLINEopt column set and some indexes are being done OFFLINE, then you could be on an edition of SQLServer that doesn’t support online reindexing. In this case, Minion Reindexing will change it to OFFLINE mode for you.  

You could also have a legacy data type in the index itself, and for versions of SQL Server under 2014, this automatically means OFFLINE mode reindexing.  These legacy types are varchar(max), nvarchar(max), text, image, and also includes, xml, and the spatial data types.  If it’s a clustered index in question, it’ll be done offline if the table itself has a legacy datatype.  This is a SQL Server limitation.