I'm no expert, but I've only reefed three times and always with loose footed mainsails on a small jboat, the outhaul is attached to the clew reefing point. Because the sail is a triangle, the outhaul is now much longer than when the sail wasn't reefed. Maybe 10 inches reefed vs. 3 inches unreefed. The angle on that 10 inches is almost horizontal or running along the boom.
As you sail with it reefed and the heavy wind gets on the sail, there is a lot of pressure on the outhaul. This is compounded by the leverage it gets when going from the horizontal direction along the boom, to the vertical force it recieves from the wind, up and away from the boom.
It's somewhat like pulling the string on a bow and arrow. It would be too tight to stretch if you tried to pull one end of the string itself, but if you grab the string in the middle of a bow and pull away from both points, you have more leverage and something has to give, the bow frame or the string. So in this case, the boom is a bow frame that isn't going to give. The bow string is the outhaul and reefed foot of the sail. The archer is the wind, pulling the reefed mainsail at the point where the reefing clew meets that reefed foot.
In our case, this force was too much. In several occations and what gave was the outhaul cleat holding the outhaul in and the back of the main came up. If it had been tied down in the middle, it's possible there would be alot of pressure on that back tie point and it could have ripped the sail. I think this is why some people don't recommend using solid ties in the middle. Someone recommended bungie cords, that may work.
We also tried lashing the reefing clew to the boom and the outhaul at the same time. this seemed to help remove pressure on the outhaul, but I don't know if we were supposed to do that.