From
http://dev.mysql.com/doc/refman/5.6/en/innodb-restrictions.html
Cascaded foreign key actions do not activate triggers
In other words, you cannot use the trigger with cascaded deleting.
Also see related bugs:
- MySQL: https://bugs.mysql.com/bug.php?id=11472
- MariaDB: https://jira.mariadb.org/browse/MDEV-19402