Drupal and event on bootstrap modal
This isn't exactly a Drupal question :)

Why not simply clear the form data as soon as you get to the AJAX success point?

Drupal.behaviors.myBehavior = {
  attach: function (context, settings) {
    //code starts
    $('#feedback-form').submit(function(e) {
      $('#feedback-form').hide().parent().append('&lt;div id="loader" class="well well-lg" style="text-align:center"&gt;&lt;h3 class="alert alert-info"&gt;Please wait ...&lt;/h3&gt;&lt;/div&gt;');
      $.post("email_us.php",form.serialize(), function(data){
        $('#loader').replaceWith('&lt;div id="loader1" class="well well-lg" style="text-align:center"&gt;&lt;h3 class="alert alert-success"&gt;'+data+'&lt;/h3&gt;&lt;/div&gt;');
        $('#feedback-form').trigger('reset');
      });
      e.preventDefault();
  })
}


As far as I can see, if you modify the code like this, it will:


hide the form in preparation to post (no change)
post, and wait for success (no change)
show the success message (no change)
clear the form immediately (modified)


Does this solve the problem, or have I misinterpreted what you need in a big way?



To answer the specific question, it's not clear to me what the problem might be, but I'm suspicious that it's caused by the on event handler being attached inside the $post function's success function. Maybe try moving it outside as shown below:

Drupal.behaviors.myBehavior = {
  attach: function (context, settings) {
    //code starts
    $('#feedbackModal').on('hidden.bs.modal', function (e) {
      $('#loader1').remove();
      $('#feedback-form').show().trigger('reset');
    });
    $('#feedback-form').submit(function(e) {
      $('#feedback-form').hide().parent().append('&lt;div id="loader" class="well well-lg" style="text-align:center"&gt;&lt;h3 class="alert alert-info"&gt;Please wait ...&lt;/h3&gt;&lt;/div&gt;');
      $.post("email_us.php",form.serialize(), function(data){
        $('#loader').replaceWith('&lt;div id="loader1" class="well well-lg" style="text-align:center"&gt;&lt;h3 class="alert alert-success"&gt;'+data+'&lt;/h3&gt;&lt;/div&gt;');
      });
      e.preventDefault();
    })
  }
}