Why is a category/ tag name prefixed to title of every page?

Question

Recently, I've noticed there is a category/ tag name "Contests" added to the title of all of my pages.

I've checked the "Tagline" under "Settings" >> "General" of course, it hasn't changed.

This is what's in header.php and I have never touched it at all:

<title><?php wp_title(''); ?><?php if(wp_title('', false)) { echo ' :'; } ?> <?php bloginfo('name'); ?> </title>