One thing that I have noticed receiving this is from unsecured websites. The page may not mean that your site is fraudulent, only a warning that it could be. I have had this on my own site when an SSL certificate has expired, invalid, or something along those lines.
Also...From Legal - Safari & Privacy - Apple - They explain what the warning is, and how to turn it off.
"Fraudulent Website Warning
When Fraudulent Website Warning is enabled, Safari will display a warning if the website you are visiting is a suspected phishing website. Phishing is a fraudulent attempt to steal your personal data, such as user names, passwords, and other account information. A fraudulent website masquerades as a legitimate one, such as a bank, financial institution, or email service provider. Before visiting a website, Safari may send information calculated from the website address to Google Safe Browsing and Apple to check if the website is fraudulent. For users with China mainland or Hong Kong set as their region in Settings > General > Language and Region, Safari may also use Tencent Safe Browsing to do this check. The actual website address is never shared with the safe browsing provider. Google (and, for users with China mainland or Hong Kong set as their region, Tencent) may also log your IP address when information is sent to them. You can disable Fraudulent Website Warnings in Safari at any time by going to Settings > Safari, then tapping to turn off Fraudulent Website Warning."