its because the menu item is a parent item for that post eg. a post within the same category.
by default WP will give the menu item a current-menu-* class because of that relantionship, which attracts the current menu item styles.
If you want a current menu item to have a hover style add this CSS:
.slideout-navigation.main-navigation .main-nav ul li[class*="current-menu-"]:hover > a {
color: #f00;
}