@xinze wang: It looks like it's all of the cells that are not connected to power and ground. Entries like

Net: _1270_/VGND 
  sky130_fd_sc_hd__conb_1/VGND = 1

Mean that standard cell instance

_1270_

VGND is isolated (fans out to 1 device, which is itself). I'm not sure how this comes about, because it indicates that those cells don't even abut other standard cells. Have you looked at the layout?