I had noticed that my profile picture also changed automatically this week. The following screenshot combo is showing the difference for my profile picture (upper half showing older ones).

[![Combo snapshots showing different profile pictures for clearkimura][1]][1]

Similar to OP, I have been:

* using auto-generated pattern image and never uploaded any personal picture;

* using same login method i.e. Google account with same e-mail address;

* using same computer and same location i.e. from home

Then this week, all of sudden, I realized that my public IP address has been assigned to significantly different ones. While it is known that the public IP address could change from time to time, it is the first time for my computer to be assigned with different range of public IP address.

The difference is significant such that it affects my login experience on Ask Ubuntu (profile picture changed) and Google Mail (precautionary two-factor authentication was activated).

**TL;DR** Significant change in public IP address might have caused changes to the profile picture, for users with auto-generated profile image.

**References**

1. [Why does my public IP address change?](https://www.quora.com/Why-does-my-public-IP-address-change)

2. [Why Do ISPs Change Your IP Address?](http://www.howtogeek.com/163747/why-do-isps-change-your-ip-address/)

  [1]: https://i.sstatic.net/ClWFL.png