On ABR for Windows Workstation, the most common causes of unexpected UAC prompts are the following:
Running an EXE directly from a browser download prompt.
Outside of Tray Tools, trying to modify settings that are classed as "Advanced System Settings".
Running an installer in user mode that subsequently triggers another MSI or EXE requiring elevated privileges.
Modifying an item that exists on the Public Desktop (e.g. a shortcut).
Using File Explorer to copy a file to a location that requires elevated privileges.
In addition to these, membership of the following groups is also known to cause problems in some instances:
Power Users
Network Configuration Operators
Group Policy Creator
As a rule, Admin By Request users who are administrators (i.e., red icon) typically do not receive ABR-style pop-ups when executing tasks that require elevated privileges. This is because the ABR client does not interfere if the logged-on user is an administrator - it simply defers to the Windows UAC pop-up. However, Admin By Request will still log everything that is done with elevated privileges.
Refer to User Account Control for more information.