I manually tracked the following footage by placing point tracks (shift+right click) and F3, all the points are green means ok. Then F9 to create camera. I shift to full mode and come out of the Lock on camera mode and pan the camera and see that some of the point tracks are behind the camera. why is it so? is it ok?

Does the camera move past these points in real life?
 
If it doesn't, then it means your calibration is not correct.  Try disabling or deleting the points behind the camera and resolve...see if that helps
 
As a general rule, you want your 3d point cloud to represent "real life" as close as possible.
 
Mike
 
 

thanx 4 ur help mike, actually I am a 3D Animator and very new to cam matching world. I will try as u said.

also, have you created a coordinate system?
 
if so then you did set a ref distance and depending on this number, you will get different solutions including some that might put your 3D points inside as well as behind the camera.
 
don;t know if you did but try to define constraints as well like a planar constraints for the points that are on the ground.