I tend to give most people a little more credit than to automatically assume it's about insecurity and/or control. (I know some people have those issues, but I don't really think they're the problem nearly as often as a lot of people assume they are.)
I tend to lean more toward assuming there are a couple of different roots of the problem:
1. The generational differences between the two women. A lot of young people don't understand why parent-generation folks think the way they do, and can't imagine that some things don't come from parents' wanting to control their grown kids. A person has to have grown kids herself to know how most mothers absolutely have no wish to control their grown son/daughter. I think, though, that this worry in the grown kids' generation is there, so they often worry a little more about someone trying to control them than is really necessary.
As a result, sometimes the younger woman will be "all ready for" dealing with the mother. Also, not being a mother of a grown son/child herself, the younger woman just doesn't know how much normal mothers respect their grown kids' independence and adulthood. She may "over-interpret" the mother's attachment to or tenderness toward her son as "being too motherly" (and kind of threatening to him). So she may think he needs her to "stand up for him" or "be on his side".
2. The fact that the younger woman comes from a different family. Often, grown kids just (naturally) fit in in their own families and with their own mother. People from the same family tend to "be cut from the same cloth". The grown son can feel both "cut from that same cloth" but also want to be separate and grown and "build a cloth" with his (for example) wife. People who don't realize that the mother doesn't (if she's normal and healthy) see it "as a competition" between "old cloth" and "new cloth" (lol) can kind of build up more defensiveness or caution than they really need to.
When all is said and done I think both women usually have the best interest of the guy at heart. (Something else, though, is that if the guy has complained about a few things his mother has done, his girlfriend/wife will already be on the alert for the behavior (when chances are he can pretty much take care of himself if mother gets too mother-y, or "whatever").
So sometimes it's not about control. It's just about people not realizing they should relax and be a little less "paranoid" about the other woman hurting the guy.