How to debug, SQL Server exception, The timeout period elapsed prior to completion of the operation or the server is not responding

I am loading a big file (~25 GB, 400 million lines) into the SQL Server 2014 using SqlBulkCopy.

My bulk copy size is 10k lines and timeout is 1 hour. The whole file is in a single transaction as I would like to store the entire file or roll back in case of failures. I have 3 indexes (one unique clustered index and other two are non clustered indexes) on this table.

Sometimes I get this exception:

System.Data.SqlClient.SqlException (0x80131904): Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding.

System.ComponentModel.Win32Exception (0x80004005): The wait operation timed out

at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection, Action1 wrapCloseInAction) at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose) at System.Data.SqlClient.TdsParserStateObject.ReadSniError(TdsParserStateObject stateObj, UInt32 error) at System.Data.SqlClient.TdsParserStateObject.ReadSniSyncOverAsync() at System.Data.SqlClient.TdsParserStateObject.TryReadNetworkPacket() at System.Data.SqlClient.TdsParserStateObject.TryPrepareBuffer() at System.Data.SqlClient.TdsParserStateObject.TryReadByte(Byte& value) at System.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady) at System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj) at System.Data.SqlClient.SqlBulkCopy.RunParser(BulkCopySimpleResultSet bulkCopyHandler) at System.Data.SqlClient.SqlBulkCopy.CopyBatchesAsyncContinuedOnSuccess(BulkCopySimpleResultSet internalResults, String updateBulkCommandText, CancellationToken cts, TaskCompletionSource1 source) at System.Data.SqlClient.SqlBulkCopy.CopyBatchesAsyncContinued(BulkCopySimpleResultSet internalResults, String updateBulkCommandText, CancellationToken cts, TaskCompletionSource1 source) at System.Data.SqlClient.SqlBulkCopy.CopyBatchesAsync(BulkCopySimpleResultSet internalResults, String updateBulkCommandText, CancellationToken cts, TaskCompletionSource1 source) at System.Data.SqlClient.SqlBulkCopy.WriteToServerInternalRestContinuedAsync(BulkCopySimpleResultSet internalResults, CancellationToken cts, TaskCompletionSource1 source) at System.Data.SqlClient.SqlBulkCopy.WriteToServerInternalRestAsync(CancellationToken cts, TaskCompletionSource1 source) at System.Data.SqlClient.SqlBulkCopy.WriteToServerInternalAsync(CancellationToken ctoken) at System.Data.SqlClient.SqlBulkCopy.WriteRowSourceToServerAsync(Int32 columnCount, CancellationToken ctoken) at System.Data.SqlClient.SqlBulkCopy.WriteToServer(IDataReader reader)

In case of exception, I do a rollback of entire transaction but then I see one more exception while trying to rollback. I didn't understand why the SqlTransaction is not in usable state as my code doesn't close the connection/transaction anywhere else.

System.InvalidOperationException: This SqlTransaction has completed; it is no longer usable.

I have debugged to some extent, in SSMS I see that there is a open transaction

select @@TRANCOUNT  ==> this gives "0" (I didn't understand why)

dbcc opentran ==> this gives me a open transaction

Oldest active transaction:
    SPID (server process ID): 58
    UID (user ID) : -1
    Name          : user_transaction
    LSN           : (159:2843368:1)
    Start time    : Dec 30 2014 11:20:13:903PM
    SID           : 0x0105000000000005150000005d28f57fd53ad8354354e02ae9881a00
DBCC execution completed. If DBCC printed error messages, contact your system administrator.

Then using this SQL script I figured out that it got stuck on the line

insert bulk dbo.#mytablename#

with status as running, timestamp exactly matching the time at which it threw the exception - 1 hour (bulk copy timeout). But I couldn't figure out why SQL Server got stuck here and why it is taking more time. Is there a way to debug this further?

SELECT 
   r.[session_id],
   c.[client_net_address],
   s.[host_name],
   c.[connect_time],
   [request_start_time] = s.[last_request_start_time],
   [current_time] = CURRENT_TIMESTAMP,
   r.[percent_complete],
   [estimated_finish_time] = DATEADD
       (
           MILLISECOND,
           r.[estimated_completion_time], 
           CURRENT_TIMESTAMP
       ),
   current_command = SUBSTRING
       (
           t.[text],
           r.[statement_start_offset]/2,
           COALESCE(NULLIF(r.[statement_end_offset], -1)/2, 2147483647)
       ),
   module = COALESCE(QUOTENAME(OBJECT_SCHEMA_NAME(t.[objectid], t.[dbid])) 
       + '.' + QUOTENAME(OBJECT_NAME(t.[objectid], t.[dbid])), '<ad hoc>'),
   [status] = UPPER(s.[status])
 FROM
     sys.dm_exec_connections AS c
 INNER JOIN
     sys.dm_exec_sessions AS s
     ON c.session_id = s.session_id
 LEFT OUTER JOIN
     sys.dm_exec_requests AS r
     ON r.[session_id] = s.[session_id]
 OUTER APPLY
     sys.dm_exec_sql_text(r.[sql_handle]) AS t
 WHERE
     c.session_id = 58;