Good questions. I think there are 2 theories on this, though others might know more:

1) The monster is from outer space. Its "egg" crash lands on earth (in the ocean near NYC). The creature is a juvenile, so when it surfaces it is freaked out and that's why it goes on a rampage. Evidence for this theory includes the final shot of the movie when you see the main characters at Coney Island on the Ferris Wheel, if you look to the far right at the beginning of this scene, you will see a small asteroid thing fly out of the sky and crash in the ocean. Also, the creators of the show have publicly stated that they designed the monster to be more like a "young" creature and that its eyes were supposed to look like a scared horse (or something to that effect).

2) The monster is from Earth and is just one of those creatures that we don't know anything about but have disturbed through human activity/pollution. This idea is thrown forth by the character who is filming the events.

I tend towards the first theory b/c of the final scene and the asteroid that hits the water. But who knows...

i agree with theory 2 kind of exept it was disterbed by a serch team in a sub looking for a fallen satalite. (falling object on far right of screen)