A common cause for this type of symptom is when external access is allowed to the cameras and an internet bot messes with the camera by exploiting the 'Hikvision backdoor' vulnerability that's in that version of firmware. There are also brickerbots that try to brick the camera, not just change the password.
External access would be possible if 'port forwarding' has been explicitly configured on your router so you can access when away from home, or inadvertently when UPnP is enabled on the router, and the camera, where it is enabled by default.
If you don't think there should be any external access, you can easily check using a service such as ShieldsUp!
GRC | ShieldsUP! — Internet Vulnerability Profiling
Do the full port scan,. not the UPnP check.
Firmware versions after 5.4.41 are not vulnerable to the Hikvision backdoor, and don't spontaneously have their passwords reset.
It might be interesting, next time this password problem occurs, to extract a copy of the configuration file, using the backdoor vulnerability.
This does not require a password to do.
Simply put this URL in your browser, replacing the IP address with that of the camera :
http://<camera_IP_address>/System/configurationFile?auth=YWRtaW46MTEK
Then zip up the file and attach it here.
We can decrypt and decode it and extract the password.
Alternatively, a common password that the hackerbots set is 1111aaaa
If that works for you - it's a reasonable confirmation the camera has been hacked.