I have two theories:
1. I was juggling a bunch of gear, some of which likely sat on/knocked against poison ivy plants. When I collected everything in my arms, the urishiol (oil) got transferred to the inside of my wrists.
2. It was hot out. I had sweat pouring out from under my gloves every time I raised my hands. That moisture represented a gap in my skin's defenses, so that's why I got the blisters there.
I like theory 2. I had a short sleeve shirt and the scratches on my arms to show that I blundered into just about everything. But I have only a few stray bumps near the scratches.
Luckily, I'm still working through the giant tub of prescription steroid cream I got in grad school for just such an occasion.