why does the bootstrap modal only show from element?

I'm developping  a Asp.net Web Application using VS2013  under framework 4.5.1 .
I want a bootstrap modal to be show when user clicks a Button , but it only show in a fewsecond ,and when I click again the modal not show anymore,
but when I using a <a> or <Input>  replace the button  it can  work fine  , Why ?  My html code like below:
<%-- <asp:Button ID="Button2" runat="server" class="btn btn-primary" Text="select" data-toggle="modal" data-target="#selCategory" /> this does not work --% <%--<a data-toggle="modal" data-original-title="Help" data-placement="bottom" class="btn btn-default btn-sm" href="#selCategory"></a> this can work fine --%> <input id="Button1" type="button" value="button" data-t ...