Currently there is no support for multiple WHEN clauses in a trigger. The WHEN clause is generally used with a simple condition to call the trigger only when necessary.
For more complex conditions use a CASE or IF condition:
CREATE TRIGGER perosn_trig AFTER UPDATE ON person
REFERENCING NEW AS nwrow OLD as oldrow
FOR EACH ROW
BEGIN ATOMIC
IF ( nwrow.person_id>100 ) THEN
insert into TRIGLOG values ('PERSON_more_than_100',nwrow.person_id,SYSDATE);
ELSE
insert into TRIGLOG values ('PERSON_less_than_100',nwrow.person_id,SYSDATE);
END IF;
END
http://hsqldb.org/doc/2.0/guide/sqlroutines-chapt.html#src_psm_conditional