Some women are always searching for an upgrade... and on another hand they are always searching for new men, not necessarily to upgrade, but to round out their team. They may have found their goalie, but now they are looking for their defenseman.
For example, she will feel lonely, feel like cuddling, and need of affection, so she'll search for one guy to handle that and keep him in that role. When she has that and feel he's secured, she'll still be with him, but then search for some side guys for thrills, aka the additional excitement of a new and unpredictable man.
In essence, she'll seek security from one man and give him affection, nice compliments, utter words of "love" etc., every now and then to keep him for emotional and physical security, but she'll have a completely different dude in which she'll show a completely different, nasty side to.
A lot of women she will have two sides she will display with a man... Miss Sweet and Miss Freak. One for the dude she wants to cuddle with and then another for the dude she knows she can't draw into a relationship, but will knock her socks of between the sheets. .