Could be that you're eating more salt with the primal food- fats, proteins, etc tend to go along with salt. That could make you thirstier. There's also some water bound up with fruit and other higher carb stuff, you wouldn't be getting that anymore.
I don't know if this is scientifically supported, but there's anecdotal reports about people feeling bloated by cheats on grains or sweets, so they can make you retain water. PB will make you retain less, so that's more water you need to take in.
Could be the humidity too, depending on where you live. When I go to drier climates, I'm a lot drier in every way, including thirst.
You could be just using up a lot of water in digestion, if you're eating less quick carbs and more fiber-based stuff and protein. That's more work to digest. Even if you're not really eating more veggies than usual, you're probably eating LESS quick-digesting food. Same thing.
Could be a lot of things.
Getting my Grok on in the Pacific Northwest.
"C is for cookie, that's good enough for me."
"Cookie is a sometimes food."
"Sometimes cookie monster eat APPLE instead of COOKIE. Sometimes eat CARROT."
-Cookie Monster, partially reformed sugarholic