Multi-master conflicts can arise when multiple nodes make changes that affect the same tables in ways that can interact with each other. The BDR system should be monitored to ensure that conflicts are identified and, where possible, applicaiton changes are made to eliminate them or make them less frequent.
Not all conflicts are logged to bdr.bdr_conflict_history even when bdr.log_conflicts_to_table is on. Conflicts where BDR cannot proactively detect and handle the conflict (like 3-way foreign key conflicts) will result in an ERROR message in the PostgreSQL logs and an increment of bdr.pg_stat_bdr.nr_rollbacks on that node for the connection the conflicting transaction originated from.
If pg_stat_bdr.nr_rollbacks keeps increasing and a node isn't making forward progress, then it's likely there's a divergent conflict or other issue that may need administrator action. Check the log files for that node for details.