In HTML, some form controls will post the form when Enter is pressed even if there is no submit button. See http://www.whatwg.org/specs/web-apps/current-work/multipage/association-of-controls-and-forms.html#implicit-submission. You got a concurrency error because, since there was no submit button, EWF was not expecting an implicit-submission post-back to occur.
I recently improved EWF's support for this situation; see https://enduracode.kilnhg.com/Code/Ewl/Group/Bill/History/a91e41468f96. EWF now throws a helpful exception if you try to build a page that includes implicit-submission form controls but no submit button.