From this in May
to this two months later.
The Horse Chestnut trees along the Ock Valley walk were in a worse state than those alongside Caldecott Rec. The one above is from Ock Valley, and the one below from Caldecott Rec.
So what is doing it?
It is definitely not drought this year. It could be a very serious condition called Bleeding Canker which kills the tree off under the bark. More likely, for most trees (and I say this after having done a few minutes research on the internet), a moth called the horse chestnut leaf miner has laid its larvae within the leaves. The leaves eventually shrivel and drop off prematurely, but grow again the following year.
you are quite right. I can see them wriggling.
I have this problem too. If you hold an infected leaf up to the light you may see a tiny little caterpillar wriggling in each brown patch. Get rid of the leaves by burning or burying them when they drop in autumn to prevent them growing into moths to reinfest next year.