Append hint with autotrace

I’m doing a direct-path insert which executes successfully. When I try to autotrace the execution, I get the ORA-12838: cannot read/modify an object after modifying it in parallel.

SQL> insert into emp_big select * from emp_big_temp;

411843 rows created.

Elapsed: 00:00:00.89

Execution Plan
----------------------------------------------------------
Plan hash value: 3203427748

-----------------------------------------------------------------------------------------
| Id  | Operation                | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | INSERT STATEMENT         |              |   357K|    45M|  1164   (1)| 00:00:14 |
|   1 |  LOAD TABLE CONVENTIONAL | EMP_BIG      |       |       |            |          |
|   2 |   TABLE ACCESS FULL      | EMP_BIG_TEMP |   357K|    45M|  1164   (1)| 00:00:14 |
-----------------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
         15  recursive calls
      31909  db block gets
      12829  consistent gets
          0  physical reads
   34685216  redo size
        845  bytes sent via SQL*Net to client
        802  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
     411843  rows processed

SQL> rollback;

Rollback complete.

Elapsed: 00:00:00.18
SQL>
SQL> insert /*+ append */ into emp_big select * from emp_big_temp;

411843 rows created.

Elapsed: 00:00:00.18

Execution Plan
----------------------------------------------------------
ERROR:
ORA-12838: cannot read/modify an object after modifying it in parallel


SP2-0612: Error generating AUTOTRACE EXPLAIN report

Statistics
----------------------------------------------------------
          0  recursive calls
       4438  db block gets
       4283  consistent gets
          0  physical reads
       7648  redo size
        829  bytes sent via SQL*Net to client
        816  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
     411843  rows processed

Questions:

Table emp_big has degree of parallelism set to 1, according to select table_name, degree from user_tables. Why does Oracle force parallel insert?
Why does using autotrace trace cause ORA-12838?