We have a website where we develop all the pages with manual HTML code, except for the /blog directory, which is managed in WordPress. A few weeks back, we encountered example.com/blog getting redirected to the /503.shtml URL, while we didn't have such a file. There is no redirection in the .htaccess like that as well. Even the /503.shtml showed the contents of our custom 404 page, as I believe the 503 page didn't exist.
Now, I'm aware of the 503 error code being a temporary server-side issue, which resolved itself shortly after. But, I'm wondering what caused the browser to redirect the /blog URL /503.shtml URL, instead of throwing 503 error on the page itself.
Is this redirection a normal behavior caused by the browser itself?