I'm not sure this applies with the "base" tag, but on links, the "target" attribute can either have one predefined keyword, or any name you want to give the new window.
The available keywords are:
- _blank: opens the links in a new window or tab
- _self: opens the links in the same frame as it was clicked (this is default)
- _parent: opens the links in the parent frame
- _top: opens the links in the full body of the window
(http://www.w3schools.com/tags/att_a_target.asp)
If you don't use one of those keywords, you can use any name you want and this name will then be used to refer to that window. This allows you to reuse a tab you opened to load a different document in it.
So by using "_new" (which is not a keyword) as the base target, you essentially say that all links must be opened in the window named "_new". At first this window does not exist, so the browser creates it (first click), and the it reuses it for all following clicks.
Use "_blank" instead so that each link opens in its own new tab.