Sounds like your settings have changed to enable
options/irc/logging/automatic-log-and-reload
Inside that button, the top row determines the default yes/no for logging that type of window, and the lowest row indicates whether it defaults to loading the last few lines of the logfile into the window each time you open it.
If you did nothing to change this setting, one cause for this suddenly being enabled is that this is the default setting, so if the rare situation happens where mirc.ini has become damaged to size zero during a Windows crash, then mirc.ini is re-created with default settings, and you would have found other configuration settings possibly changed back to defaults, including not remembering which of your additional scriptfiles are loaded, or custom event colors.
