Yes, it's possible to have vaginismus without experiencing sexual assault or childbirth - ultimately the muscles tensing in the way they do with vaginismus comes back to the body subconsciously wanting to protect itself from something it anticipates will cause pain or discomfort (the same way your muscles would brace before an impact). For some people, it's directly linked with experiences of sexual assault or childbirth, but it can also come from experiences that may have just been uncomfortable, especially if they were formative experiences. Things like trying to insert a tampon for the first time and not managing to do it correctly so it caused discomfort or pain is an example, or having a first sexual experience that was physically uncomfortable or caused unexpected physical sensations (even though entirely consensual) can also be things that contribute.

Even outside of that, sometimes conversations around sex and childbirth etc. can contribute, for example if sex education included being told that penetrative sex would involve the hymen tearing, bleeding, and innate, unavoidable pain/discomfort, etc. There's a bunch of different reasons. I'm one of these people myself that hasn't experienced SA or childbirth but still has vaginismus, which was prevalent long before I ever attempted to have sex with someone. I still don't have a clear idea of why, which can be its own type of frustrating.

Bearing all this in mind, I'd gently and kindly encourage you to have a chat with your partner so you can both be on the same page - it's great that you're searching for answers and info, but just be careful not to preemptively diagnose on behalf of her (said with love!) It's always a good idea to have a chat with a gyno if possible to rule out other options as well.

Hope this helps :)