I've no idea why it only happens on AB but it sounds like a malware infection to me.
Download, install and run the free version of Malwarebytes Anti-Malware:
https://www.malwarebytes.org/antimalware/
(NB: At the end of the installation process you'll encounter a pre-ticked box offering you a free trial of the 'Premium' version. Deselect that option before continuing).