enq: TM - contention observed for delete statement

Hello experts,

                       I am trying to understand below scenario

SQL> CREATE TABLE supplier
( supplier_id number(10) not null,
supplier_name varchar2(50) not null,
contact_name varchar2(50),
CONSTRAINT supplier_pk PRIMARY KEY (supplier_id)
);
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Table created.

SQL> INSERT INTO supplier VALUES (1, 'Supplier 1', 'Contact 1');
INSERT INTO supplier VALUES (2, 'Supplier 2', 'Contact 2');
COMMIT;

1 row created.

SQL>
1 row created.

SQL>
Commit complete.

SQL> CREATE TABLE product
( product_id number(10) not null,
product_name varchar2(50) not null,
supplier_id number(10) not null,
CONSTRAINT fk_supplier
FOREIGN KEY (supplier_id)
REFERENCES supplier(supplier_id));
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Table created.

SQL> INSERT INTO product VALUES (1, 'Product 1', 1);