Weird behaviour with high gravity

Unity Engine
1

I am making a 2d game and when jumps in the z direction using rigidbody.velocity=transform.TransformDirection (0, 0, 100); and set gravity to physics.gravity(Vector3(0.-150,0)); in the start it jumps in the z direction but sometimes, randomly it seems, it goes up in the y direction in the middle of the jump without any reason, but most times the jump goes well.

2

Any reason why you need such crazy gravity?

3

So that when a moving platform goes up the player doesnt jump up in the air when goes down again