RS-485: what happens when bus contention occurs
RS-485 does need to share a reference for communication. The allowed maximum difference between grounds of different nodes is +/- 7V, so without providing a common ground reference between nodes, the voltage might be exceeded and nodes could get damaged or at least they would not communicate properly. The common ground reference might be a separate wire between devices, or for example devices could be referenced via mains earth.
When two devices drive a bus to opposing state, a lot of current flows and the voltage level is indeterminate. Fortunately, the datasheet of the driver you have says the outputs have overload protection.
Standard protection procedures include series resistances or self-resetting polyfuses, but in general, the drivers are able to protect themselves from occasional bus contention.