Why is it that sometimes users have their new profile picture automatically added to their account? Occasionally, if a user uploads a new picture, the older image overrides the new one, but there's no error message or indication of why this happens.
Zendesk does not automatically override an image which you upload on your profile, except in the instance where you may already have a Gravatar account. Gravatar is an avatar site linked to Wordpress where you can upload your picture as an avatar and it will then populate any other website which uses profile pictures automatically for you.
Gravatar will override the image you load into Zendesk with an old image which you may have linked to them. If users have interacted with Wordpress in the past, they may have unknowingly signed up to this as well.
To confirm if you have a Gravatar account, simply go here to check: http://en.gravatar.com/