Why is MVC button link inserting incorrect url - asp.net-mvc

Question

I have a simple mvc login page with a button generating an incorrect link. When I hover over the button, the link, which I believe is the default form action which in turn is the current page, is /resource.ashx?action=login&controller=user. It should be /action=login&controller=user. The url of the page is
http://localhost:57505/User/Login.
Can anyone explain how I would go about determining where / why / how resource.ashx is made part of the form submit action? and more importantly how to fix?
I have considered routing, and javascript files which may be modifying the html but nothing strikes me as a culprit.
The cshtml code is as follows:
#model MyCompany.MyApp.Website.Models.User.LoginViewModel
#{
Layout = "~/Views/Shared/_LoginLayout.cshtml";
ViewBag.Title = "Log in";
}
#using (Html.BeginForm(new { ViewBag.ReturnUrl }))
{
#Html.AntiForgeryToken()
#Html.ValidationSummary(true)
<h1>Welcome to My Website</h1>
<div>
#Html.TextBoxFor(m => m.UserName, new { placeholder = "Username" })
#Html.PasswordFor(m => m.Password, new { placeholder = "Password" })
<button>Login to MyWebsite</button>
<div>
#Html.CheckBoxFor(m => m.RememberMe)
<label for="keep">Keep me logged in for 5 days</label>
</div>
#Html.RouteLink("Password recovery",
routeValues: new { controller = "Password", action = "Forgot" })
<br />
</div>
}
I should also add that this problem occurs on an instance of the solution which I branched in tfs express 2013. there is an extra level in the directory structure than in the original which works although I could not explain why that matters. the problem is the more vexing because the base code works, while the branched code does not. and I copied the working code into the branch to make sure everything was exactly the same, yet the application is adding this additional element, resource.ashx to the computed url. I also set the directory structure the same, but no success.
As a second addition, to answer rowan's question about routing, this is what the application has. I have inherited it, so I may not be able to answer / justify all questions about decisions made.
public class RouteConfig
{
/// <summary>
/// Registers the routes.
/// </summary>
/// <param name="routes">The routes.</param>
public static void RegisterRoutes(RouteCollection routes)
{
routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
////routes.LowercaseUrls = true;
// Client website routes
routes.MapRoute(
"user-activation",
"user/activate/{token}",
new { controller = "user", action = "activate" });
routes.MapRoute(
"password-reset",
"password/reset/{token}",
new { controller = "password", action = "reset" });
routes.MapRoute(
"Default",
"{controller}/{action}/{id}",
new { controller = "Home", action = "Index", id = UrlParameter.Optional });
}
}

Answer 1

I figured out the problem. a third party control, which was not even being used or referenced, but which nonetheless was sitting around in the solution, was the cause of the unexpected route which caused the unwanted url on my login page. deleting the related dlls fixed the problem. however, I do need that component, so will have to work out a solution.
special thanks to rowan for getting me on the right path. all I needed to do was inspect the RouteTable in the debugger, and it became obvious what the cause and culprit were. previously I had relied on eyeballing the code, but it was doing something more than I expected. hence the need to inspect the runtime state - not just design time.