If you attach the first net to the bus, there's a variable set to the
bus object called "ripperdir".
If you have a vertical bus and you connect your first net to the upper
half, the busripper is drawn below the net.
If you draw the first net into the lower half, the busripper is drown
above the net.
If you have a horizontal bus, the same applies to left and right.
See documentation --> file format --> bus, for the variable.
And o_net_add_busripper() in file gschem/src/o_net.c for the decision of
the ripper direction.