Final answer:
Malware is an operating system problem that might cause the desktop background to change unexpectedly.
Step-by-step explanation:
The operating system problem that might cause the desktop background to change unexpectedly is malware.
Malware, or malicious software, is software that is specifically designed to damage or disrupt a computer system. Some types of malware are designed to change the desktop background as a form of visual clue that the computer has been infected.
For example, a specific type of malware called ransomware is known to change the desktop background to display a message demanding a ransom in exchange for restoring access to the computer.