What do "HTTP Processor" events mean?
"HTTP Processor" events are not coming from rules (Categories/Sub-Categories).
Basically, these types of events can occur when the HTTP request is malformed in some way.
An event is written and the indication of its cause appears under the "error" section.
We believe that the request was not completed. (Perhaps the browser was closed prematurely while doing the request.)
You may disable this by checking "Don't Check Invalid Requests" under
"Default Security Profile" -> "Advanced Settings".
Mind that if doing so, the IIS will receive every malformed request instead.