It does that if the HTTP request to load in the captcha fails. You should be able to see the failed HTTP request in your browsers developer console (press F12) under the network tab (may also show in the console). You can turn off the ajax loading of captcha within CB AntiSpam, which should avoid this but will also reduce captchas effectiveness against bots. Does registration also do this? Strange that it fails every other reload. Have added a feature ticket for it to retry if it fails instead of give up after 1 attempt, which may help prevent issues like this from happening.
forge.joomlapolis.com/issues/7031
Do you have a clone of your site where this issue is present? If so please PM backend super administrator login credentials. If not you can PM live credentials, but what I'll be able to test will be extremely limited due to it being live (please generate a full backup first if you choose to do this).