Its as Chris explains.
We were using an Arbormaster technique where you plunge through the face cut, out the back of the tree, even with your face's horizontal, than plunge in again, parellel and below the first plunge cut about 1/2". You can do it a third time, too. You stack wedges with the wood between them, keeping them from slipping.
I'd just done it on a smaller tree, plunging in both times from the face cut. Duane did it, but on this bigger tree, he should have done the second plunge cut from the rear, ensuring that he'd be more level where the wedging was going on, but instead he was 1/2"+ on the face side and slightly sloped, not dead horizontal, coming out with 1/4" of wood on the back side between plunge cuts. That was the problem. The sawyer then cuts the wood on both sided of the wedge, and commences pounding the wedges. Seemed like it was going to go okay, until the wedge backed out, and the tree set back.
I should have set two more wedges immediately, on the sides of the stacked wedges, but didn't, as normally you don't have to. Its a good technique to do so, just like backing up a hydraulic jack. Lesson learned.
I was going to suggest that he go from the back for his second cut, but since we don't have helmet communication to simply discuss stuff without yelling, waving arms, and blowing concentration, I didn't. I'll have yet another example for the big boss as to why we should have the radios.
We've pounded over some hard back leaners with three wedges stacked in three bore cuts before without any problem, just a lot of pounding.
Duane said things happening like that every once in a great while keep us from getting overly confident. True story, that.