Not being a skier, I'm not familiar with how these normally mount, so I'll ask:

How do the bindings normally mount to the ski? Are they done with screws that thread in from the binding into the ski? Thru bolts that use a binding post from the bottom and a machine screw from the top?

My guess as to what went wrong from your post is that the screws you used to mount the bindings to the wood failed, because the amount of force on the screws caused them to pull out of the wood. Also, it appears that you mount the wood only in the middle, which means that the two ends are free to flex up and down, which will cause two scenarios: as the wood flexes up on both ends, you are compressing the bindings against the boot, putting lateral force on the screws, which will elongate the screw hole, and make it loosen over time. As the wood flexes down, you are lengthening the space between the bindings, which puts lateral force on the screws the other way, and could cause your boot to come out from them.

What I would recommend is to get longer screws that will pass straight through the wood and fasten to the ski just as it originally did (either threading into the ski or with binding posts that pass thru the ski). Oversize the holes in the wood, so you don't thread into the wood at all, and it can move freely (it will stay in place when you tighten the screws). This will reduce the flexing of the wood, while preserving the original mounting locations, which are presumably strengthened for mounting. The downside (or possibly upside?) to this is that that big piece of wood in the middle is going to reduce flexibility of the ski in the middle.

Another question I have - what is the purpose of this? Just to gain an extra 3/4" height? Does it improve the performance in some way?