I'm not positive, but I'm pretty sure it's the cobwebs.
I often see them on his legs visibly when we get back to the barn. Also, if we ride in the fields or arenas (even the grass arenas) he has no problem. If it's rainy and the spiders aren't out, he has no problem. It's just days we're going through tons of cobwebs that he has issues. Now, it's possible there's something else that's correlated with cobwebs...
However, even with the ones I go through, they often continue to itch long after I've removed them from my arms. And bits of the strands hang on in various places.
There aren't a lot of bushes to push through. Most of it's open, between trees. But these cobwebs are HUGE, and they come out of nowhere, hanging between widely spaced trees.
It might not be the same where you are? I know in Colorado I NEVER had a problem with cobwebs. Here in Virginia we're passing through a new one every few minutes. It's gross.