> A detail record can not exist without a master.

Exactly my point! But the master record is actually being deleted
before the detail.

> the rule says that when a record is deleted from T1,
> any records in T2 that point to T1 must be deleted.

However the record from T1 is being deleted first, followed by the
record from T2.

> Your before delete trigger raises the exception.

It is raising the exception because the record in T1 does not exist
anymore.

Isn't this a violation of referential integrity?