Because the verb "halljátok" is in the definite conjugation, that is, it has an implied object. It is transitive, if you like.
hall-JÁ-tok - "já" is the definite part in this case.
The indefinite version would be "hallotok":
hall-o-tok - I guess the "o" is just a buffer sound.
Here is the whole table for you, indefinite-defnite:
hallok - hallom
hallasz - hallod
hall - hallja
hallunk - halljuk
hallotok - halljátok
hallanak - hallják