ur checking password strength on server side by textchange event that's why autopostback is ocured except that add RegularExpressionValidator
for example
<asp:RegularExpressionValidator ID="revPassCheck" runat="server" ErrorMessage="Password lenght must be between 6 to 15 characters."
ControlToValidate="txtPassword" ValidationExpression="[\s\S]{7,15}"></asp:RegularExpressionValidator>