It's most likely to be a video codec crashing on certain files, or you've found a plugin or shell extension bug with certain files.
See FAQ: Crash/exit/100%CPU when viewing a certain directory for advice.
It sounds like the crash is happening when the viewer plugins are unloaded, which happens about 10 minutes after they are used. So that you don't have to wait 10 minutes to try every configuration change, you can tell Opus to unload all plugins (which aren't in use) immediately using this command:
Show FLUSHPLUGINSThe first thing I would try is disabling all plugins, after making a note of which were initially enabled, then see if the problem still happens. If it doesn't, turn some back on and try again, until you are back to your original configuration.