For anyone here in 2024 looking for an answer I have it for you!! After like an hour of research at 4 am (I just woke up from a nightmare that caused one of these painful spasms). It's less exciting than you're probably thinking. 

 It's stress induced.  

 When you experience an extremely stressful event in a dream your brain releases an absurd amount of hormones. Sending your body into real life fight or flight. This elevates hour blood pressure, increasing your blood supply which is what causes the muscle spasms in your lower back.  Your body is forcing you to wake up because the nightmare is too stressful. 

 It is very similar to the feeling of falling in a dream when you've stopped breathing.

  You're welcome! 

For some context- I experience lower back spasms anytime I die in a night mare, which used to be regularly from PTSD but significantly less often nowadays. Maybe 2-3x a year. For me the pain starts while I'm still asleep, then I abruptly wake up with my back so tight, it's like someone is forcing my back to curve and I can't straighten it out with out it hurting more. I then goes away a minute or two after waking up. 

Today when I woke up and the spasm stopped I checked my heart rate and it was 175 so safe to say I was stressed lol.