Looks like a leaf miner infestation. I see small brown tracks and then a large brown area which becomes so thin that it tears open to a large hole.

It looks like mechanical damage to me. I've seen similar damage on a plant near the entrance to a racquetball court: everyone was taking a swing at it as they entered or left! :)
If it is near a path, especially where children regularly pass, something similar could be happening. High winds could also do that, though usually more on the upper regions of the plant.

Looking at the top picture, I would say Leaf Miner. Its not so obvious on the lower picture, but you can see the tell tale trails on the top one if you expand it.

Tugbrethil, it's definitely not mechanical. The tree is very much isolated away from anyone. Strong winds could be a possibility though.

I see what you mean, Cottagekaren, but I am still not convinced. Instead of the spiraling and looping trails of leaf miners, I am seeing the linear and broken linear signs of folding and ripping. Treetop, do you have a young dog who might be chewing on the leaves? The reason I ask is that the pictures appear to show most of the damage near the bottom of the tree, which is really weird for wind damage.

There are different leaf miner patterns as per different leaf miner species. There's one that Tug is referring to looks as if something is playing "connect the dots" within the leaf. There is another type of leaf miner that will cause the damage you see here. Some of the larvae start their mining within the leaf then for one reason or another die shortly after hatching which shows as spotty brown areas. All one has to do is type in "leaf miner images/damage" to see the variety of patterns they make on different types of leaves.

Well, I did that, and I only saw one image that looked anything like that, on a honeysuckle leaf:
http://www.alamy.com/stock-photo-leaf-miner-paraphytomyza-spp-damage-to-honeysuckle-lonicera-spp-leaf-9023403.html
I checked for other images of damage by that species of leaf miner, especially on Magnolia leaves, and all of those images, as well as other species' damage, were as I described--high efficiency, curvilinear fractal patterns--though admittedly differing slightly in detail. I'll grant a faint possibility, but I'm still not convinced.

Thanks for your comments guys. Update. I have just realised, we did have the most horrendous hail storm here that lasted for about ten-fifteen minutes. I'm thinking perhaps that may have caused the damage. I didn't connect the hail and the damage at the time.