in this case "ng" is used as an object Tumatawid kami NG tulay ...You can also say Tumatawid kami SA tulay ( sa is used as a direction) .. ***Bakit SILA tumatawid ANG Ilog ??? It should be Bakit sila tumatawid ng/sa ilog..You cannot have two topics "sila" and "ang ilog"
Ang is used to mark the topic (known information), while ng is used to mark the comment (information about the topic). It is somewhat similar to how "the" & "a" are used in English (compare "the dog" vs "a dog")
For example, the sentence "Kumagat ang aso ng pusa" may be translated as "The dog bit a cat," while "Kinagat ng aso ang pusa" may be translated as "A dog bit the cat."