Since you want to do client side validation using javascript you do not need CausesValidation = "True" and AutoPostBack="True". These are server side validation property.
Also you have use OnTextChanged="javascript:validatePwd();". This is server side event which will cause postback.
You can modify your code as
<asp:textbox id="txtPwd" runat="server" textmode="Password" onblur="return validatePwd();" maxlength="12" xmlns:asp="#unknown">
or
<asp:textbox id="txtPwd" runat="server" textmode="Password" onchange="return validatePwd();" maxlength="12">
using onblur will cause validation when textbox leave focus and
using onchange will cause validation when textbox value is changed.
Also you can make your javascript validation simple by doing most validation with txtPwd textbox onblur or onchange. And just do comparison validation "You did not enter the same new password twice. Please re-enter your password." onchange of txtRePwd textbox