I can confirm that it is probably the bot programs. I have 4 accounts. Two that I botted with so far has received the warning, so I stopped botting wit them. One of them was a brand new account I cretaed last week and did NOTHING but bot. My other two accounts I just spoof and snipe and no warnings for those accounts. Once I get that warning, I know never to bot on that account ever again.
I have two accounts I used to use for a personal map that got captchas all the time...no warnings playing legit on a phone. One account I bought on eBay dirt cheap, clearly bottled and getting warnings. My own bot account got warnings, which went away after 30 or so days, I play it legit now and no warnings. My 100% legit account has never had a captcha or a warning, nor have any of my legit friends accounts. I know that's only about 20 accounts total, but I think that's representative of the people here too. For the most part, they seem to have a good algorithm for flagging non human players right now. Always fun to watch as time goes by.
The account from eBay had the warning go away after about a week of playing legit, and changing the email and password associated. Fingers crossed it lives.