That is a fairly typical experience. It is all automated in that if you are paying off the card on time they automatically generate the letter asking if you want the increase. However, before they actually give it to you they check your credit rating. Whether a call center in india makes that decision or not, i don't know. I would tend to doubt it. But it is another automatic decision in any case. they just run your credit report and if it is above some number you get it, and if it is below that number you don't.
You might want to check your credit report to see if any bad marks went on it recently. You are entitled to a free credit report every time you get denied for something.
I have also been pretty happy with https://www.creditkarma.com/ It is free and you can keep track of your credit score at all times on there and see what is affecting it and even file a complaint to have a bad mark removed if you don't feel it is accurate.