What causes power to enter ground?
If you have a multiple ground rods then that could be causing the issue. Everything should be bonded together; if they are not bonded then that could be the issue. 

http://www.reddit.com/r/electricians/comments/23jjlu/hi_reddit_is_it_a_bad_idea_to_ground_an/

34 min video on grounding https://www.youtube.com/watch?v=Ypo99VRxT44