Centrifigal Force?

Gravity?

But on a serious note, divination is an occult tool and therefore by it's very nature is reaching out to the spirit world. Something very well could be pulling on it. Should you be surprised?

I was thinking Gravity was constant and when swinging and on the way up. There would be more pull on the line. Then momentarily, it would go torwards weightlessness.
May be Centrifigual that pull on a pendullum differently than other stationary objects.