Transmission lines can be terminated at both ends. In this case, the source termination also provides DC load bias, as the transmission line is AC coupled.
The 50 ohm resistors to ground are the 100 ohm differential termination of the transmission line with 0V bias.
Note that this diagram is only an example, and the actual circuit may be different ...
I think this picture in the guide from MAXIM is self-explanatory: -
one resistor at one end of the common shared multipoint cable and one resistor at the other end.
don't use star-cabling techniques - the multi-drop cable has to be used and physically, it can snake this way or that way but keep tee-off point lengths to a minimum.
It's all about preventing ...