I'm curious as to how/why they chose who would participate in the Battle Tree. Grimsley, Sina, and Dexio are just such odd choices.
I have to agree with you. It could've just been random choices, or they may have taken these choices based off of polls no one really thought about in Japan.
We now know Anabel, Colress, Sina, Dexio, and Grimsley, the 5 opponents that hadn't been previously revealed to us. I might have reasons for as to why they might have put these 5 members in here:
Colress - Colress was such an odd character, completely shrouded in mystery when we saw him in B2W2. Plus, he was very popular back when B2W2 were the newest games.
Sina and Dexio - We never got to battle them in XY, and I think a lot of people would've liked too. We'll be able to see what Pokemon they used back in their journeys before XY, and seeing as they have quite a bit to do with the story, we'll likely learn a bit more about them.
Anabel - Anabel was a very popular frontier brain in the days of Pokemon Emerald, and I'm sure, for anime fans, in the Battle Frontier season, her very unique presence in the couple of episodes she was there made her stand out more than any other frontier brain.
Grimsley - I'm having quite a bit of trouble with this one, but a lot of Pokemon fans I have talked to who have played BW and/or B2W2 say his quotes were very intriguing, and made his character seemed fleshed out from just a few of the things he said.