It has to be the system reacting to logging in from a different IP. The first time this happened to me a week ago I had brought my laptop to a family member's home and got the message. Now, just recently I got the message trying to log in from a coffee shop. I checked the time that the e-mail was sent to me about the change and it was the exact time I tried to log in. Now that I know the reason for the change I am much more relieved someone isnt constantly trying to hack my account.