Login causes new window in browser

login

#1

I have two twitter logins on different servers using identical twitter code. One is on https secure server. Don’t know if that makes a difference.
On https, as soon as I click twitter login, it opens a new browser window, while on the other, the same window is used.
What controls whether or not a new window is used?
https://festivalpassports.com/hidden/live.php
http://cryptic.pics