Unable to redefine a large table with DBMS_REDEFINITION

BEGIN

DBMS_REDEFINITION.START_REDEF_TABLE (

  uname         => 'xxxx', 

  orig_table      => 'BIG_TABLE', -- range interval on date

  int_table       => 'BIG_TABLE_TMP', -- changing to range interval on new key

  options_flag     => DBMS_REDEFINITION.CONS_USE_ROWID,  -- has no pk

  orderby_cols     => 'PCI',

  continue_after_errors => FALSE,   

  enable_rollback    => TRUE);

END;

/  

Fails after running for several hours with :

ORA-1653: unable to extend table ENCTR.SYS_RMTAB$$_H137121 by 8192 in tablespace SYSTEM

On demand

MV owner.BIG_TABLE_TMP was not refreshed successfully.

Number of MV refresh failures: 1.

Encountered error ORA-12008.

1. Why is this job writing to SYSTEM?
2. SYSTEM is defined with autoextensible and there is free space on the mount so why is this small write failing?