
So this is something new that I haven't seen happen on Twitch before via their app. I loaded up the new Twitch Andriod app on bluestacks to test it out to verify. Bluestacks is a android emulator, which is very handy for testing apps.
After you login (I used a freshly created Twitch account, this may make a difference), you have to pick 3 games to follow. For my test I picked EVE Online, Star Citizen and Elite Dangerous.
Here are the listed streamers that the app automatically followed for each game:
Elite Dangerous Auto Follow Streamers:
Star Citizen Auto Follow Streamers:
EVE Online Auto Follow Streamers:
Twitch Support tweeted out the following:
Interesting keywords here is "new-user onboarding flow" which I believe for the app is to get users to quickly see the benefits of following games and streamers.
At first glance it seems to pick channels with the most viewers
Update 7/7/2017 - Its picking the top three streamers in the game directory now, not most followers.
Update 7/25/2017 - You need at least 10 Viewers to be included in the top 3 of the game. If the top channels are all under 10 viewers, no one will be auto followed.
Tip:
Update 7/25/2017 - The tip above is no longer available, Twitch removed Vodcast streams from follower onboarding.
Twitch Support tweeted out the following (7/24/2017):

How do you feel about the Twitch app basically follow botting certain channels? Leave a comment below, thanks!