CAS Security Check in URL?

I have a legacy web form (form code embedded in a page on my site). The thank you page for that web form is an Infusionsoft hosted order form. When I fill out the form and click “submit”, I get a screen that says “login to Infusionsoft” and the URL shows some sort of CAS Securtiy check. Here is the full URL:

Any idea why it is behaving this way and what the spring CAS Security check is??

Thank you!

@Paul_Conn, can you provide a link to your order form? You can private message me if you don’t want it public.