Version:           2.3.0 (using KDE 4.7.2) 
OS:                Linux

In the SQLite DDL:

DROP TRIGGER IF EXISTS move_tagstree;
DELIMITER //
CREATE TRIGGER move_tagstree AFTER UPDATE OF pid ON Tags
 FOR EACH ROW BEGIN
                DELETE FROM TagsTree
                    WHERE
                    ((id = OLD.id)
                    OR
                    id IN (SELECT id FROM TagsTree WHERE pid=OLD.id))
                    AND
                    pid IN (SELECT pid FROM TagsTree WHERE id=OLD.id);
                INSERT INTO TagsTree
                    SELECT NEW.id, NEW.pid
                    UNION
                    SELECT NEW.id, pid FROM TagsTree WHERE id=NEW.pid
                    UNION
                    SELECT id, NEW.pid FROM TagsTree WHERE pid=NEW.id
                    UNION
                    SELECT A.id, B.pid FROM TagsTree A, TagsTree B
                    WHERE
                    A.pid = NEW.id AND B.id = NEW.pid;
 END
//
DELIMITER ;


In "AFTER UPDATE OF pid ON Tags", the "OF pid" part is not supported by MySQL 5.1 (can't specify a particular field that is updated, only that a record has been updated as a whole)

Reproducible: Didn't try

Steps to Reproduce:
New SQLite DB - inspect the DDL

Actual Results:  
Don't know - I migrated from an older version previously migrated to MySQL. Run through a comparison of the SQLite DDL and the MySQL one, where it appeared that several triggers were missing in the MySQL verison

Expected Results:  
Using a MySQL back-end, the logic of changing a parent tag is seems to function well in the code, without the need of this trigger.

I suggest to remove this trigger.