Ok, i figured out my problem...
I'm initializing bootstrap's tooltip in $(document).ready():
$(document).ready(function ()
{
$(".bsTool").tooltip();
});
Everything ok, but... since i'm using UpdatePanels (ASP.NET) and my content is show/hidden via async postback, $(document).ready() doesn't get called!!!
In order to initialize the tooltip plugin on every async postback (and normal postbacks), i added it to a page load function:
function pageLoad()
{
$(".bsTool").tooltip();
}
pageLoad() function is called by ASP.NET Ajax on the web page's first load, but also on each async postback. Therefore the tooltip plugin is registered again after the async postback.
Problem solved :)