Bulk SQL INSERT into Azure SQL Database using spark causes blocking/contention?
You could be hitting the Resource Governance limits for the Azure SQL DB.   Query the sys.dm_db_resource_stats while running the insert and you will see more detail on what is happening.
Possible workarounds could be to load into TempDB first of all and then process into your main DB (TempDB has higher resource governor limits) or scale up for the bulk load then scale back down afterwards.   Also consider partitioning of the table you are inserting into and consider the tips here