Some websites will detect a Burp proxy and act accordingly. If you did your initial page load with any kind of integration like that, that's why the WAF may hav

Some websites will detect a Burp proxy and act accordingly. If you did your initial page load with any kind of integration like that, that's why the WAF may have blocked your request. I don't know exactly how they do it (my guess is fingerprinting the TLS handshake and TCP packet patterns), but I have seen several services do a great job at blocking any kind of analyzing proxy.