Surprisingly, I get this fairly often on my mobile. I'm not quite sure what's changed with the last version (or couple) of Android to trigger this. Over-secure, perhaps?
Android mobile browser requires the full signature chain of authorities who signed the site certificate[0]. It is done to avoid calls to CA servers and reduce network use on mobile devices.
If a person does not concatenate authorities signatures to their site certificate, Firefox and mobile Chrome will alert, desktop Chrome and Safari won't.
