Why is ASP.NET Web Api's Dependency Resolver being disposed?

I have a custom IDependencyResolver:

internal class NinjectResolver : IDependencyResolver
{
     private IKernel _kernel;

     internal NinjectResolver(params ApplicationModule[] modules)
     {
          _kernel = new StandardKernel(modules);
     }

     public IDependencyScope BeginScope()
     {
          return this;
     }

     public object GetService(Type serviceType)
     {
          return _kernel.TryGet(serviceType);
     }

     public IEnumerable<object> GetServices(Type serviceType)
     {
          return _kernel.GetAll(serviceType);
     }

     protected void Dispose(bool disposing)
     {
          if(disposing)
          {
               if(_kernel != null && !_kernel.IsDisposed)
               {
                     _kernel.Dispose();
                     _kernel = null;
               }
          }
     }

     public void Dispose()
     {
         Dispose(true);
         GC.SuppresFinalize(this);
     }
}

I am registering it like this:

public static void RegisterModules(HttpConfiguration configuration, params ApplicationModule[] modules)
{
     _resolver = new NinjectResolver(modules);
     configuration.DependencyResolver = _resolver;
}

Everything seems to work fine at the beginning. However, after some minutes (see NOTE) my NinjectResolver is disposed, and therefore begins to throw NullPointerException when trying to resolve a type (this is because I am setting _kernel = null in the Dispose method).

A way to solve this is to keep the reference alive and not set it to null when the resolver is disposed. But why should I?

I mean, why is my resolver getting disposed just like that, from one minute to another? Nowhere in my code am I disposing it explicitly, so it must be the Web Api framework doing it for me.

NOTE: Sometimes it takes a couple of minutes, and sometimes just seconds. It's completely random.