ASP.NET MVC Some Data Annotations validation showing on page load in IE

I have a very strange issue with IE and ASP.NET MVC 4 Data Annotations.

Some of them seem to be showing their server side error messages on the initial page load only in IE (9 and 9 in 8/7 standards mode). The ones affected seem to be some StringLength and RegularExpression but not all I have other Data Annotations e.g. Required which are not showing their messages.

I have customised how validation messages are inserted client side so they are definitely server side errors.

Does anyone have any idea why this might happen? I am completely confused.

I have tried calling ModelState.Clear() in my action method but this does not seem to resolve the issue..

I am using the P-R-G pattern but the G part which renders my form is a child action, could this be the issue?

Code in response to comment:

    public ActionResult MyForm()
    {
        var cp = GetComponentPresentation();

        var model = new MyFormModel(cp);
        model.ComponentId = cp.Component.Id;
        LoadModel(model);
        return PresentationView(model);
    }

    private void LoadModel(MyFormModel model)
    {
        string titleTypesTcmId = ConfigHelper.TitleTypesKeywordId.FormatWith(ConfigHelper.PublicationId);
        model.TitleKeywords = TaxonomyService.GetTaxonomyKeywordChildrenNames(titleTypesTcmId)
                                .Select(x => new SelectListItem() { Text = x, Value = x }).ToList();
    }

The View Html is 250 lines and I don't have time to anonymise it. So here is a snippet of one of the text boxes that seems to be failing validation.

<div class="row">
    @using (Html.BeginForm("MyForm", "MyFormController", FormMethod.Post, new { id = "MyForm", @class = "form ajax", data_ajax_error=Html.GetPlainTextValue(Model.Component, "error_message") }))
    {
            <input type="hidden" name="ComponentId" value="@Model.Component.Id"/>
            <fieldset class="paddingright">
               <legend>Your Details</legend>
               <div class="row field">
                   @Html.LabelForRequired(m => m.NationalInsuranceNumber, Html.Resource("Labels_Form_NINumber"))
                   @Html.TextBoxFor(m => m.NationalInsuranceNumber, new { placeholder=Html.Resource("Placeholder_Form_NINumber") })
                   @Html.ValidationMessageFor(m => m.NationalInsuranceNumber, null, new { @class = "error"})
                </div>
                <!-- SNIP -->
                <div class="submit-container no-js-hidden">
                   <button type="submit" value="Step2" class="btn-green customsubmit" name="FormButton">
                       <span class="content">Next step &gt;</span>
                   </button>
                </div>
             </fieldset>
     }