Button behaving weird

I have this code to add fields dynamicly and out of nothing the button to add new fields start behaving weird. I was working fine and out of nothing it started to process the form like the submit button. I tried setting the button as button but then it stop adding the fields.

The html:

    <form method="post" action="setup5.php">
        <div class="input_fields_wrap">
            <button class="add_field_button">
                Add More Fields
            </button>
            <div>
                <input type="text" name="mytext[]">
            </div>
        </div>
        <div>
            <input type="submit" value="Next" />
        </div>
    </form>

The JS:

<script>

$(document).ready(function() {
    var max_fields      = 10; //maximum input boxes allowed
    var wrapper         = $(".input_fields_wrap"); //Fields wrapper
    var add_button      = $(".add_field_button"); //Add button ID

    var x = 1; //initlal text box count
    $(add_button).click(function(e){ //on add input button click
        e.preventDefault();
        if(x < max_fields){ //max input box allowed
            x++; //text box increment
            $(wrapper).append('<div><input type="text" name="mytext[]"/><a href="#" class="remove_field">Remove</a></div>'); //add input box
        }
    });

    $(wrapper).on("click",".remove_field", function(e){ //user click on remove text
        e.preventDefault(); $(this).parent('div').remove(); x--;
    })
});

</script>