Why did heck tate insist that bob fell on his own knife?

Because he did not want any more trouble in Maycomb pertaining to Tom Robinson's case. He knew that it was improbable that Jem could have killed him, considering he is half of Bob Ewell's size and had just broken his arm. Although it could be inferred Boo killed him, he did not want to trouble him, because he didn't do anything wrong. He saved the kids, and did the town a favor.