I'm not sure if you're asking if the nearly constant severe lower back pain is common with dysautonomia, or if the chain of events of pain setting off a "crash" of worsening autonomic symptoms is. I would say that definitely the latter is something that sounds pretty typical, while the former is not. However, you have to take that with a grain of salt because I don't have a lot to draw on when it comes to large fiber/motor neuron involvement, so "typical" is relative to my experience.
At any rate, sticking with what I do know, there are a variety of triggers that can set off a flare up of autonomic symptoms. Different people find they are sensitive to different stressors, but common ones include flying on a plane, severe pain, bodily injury, invasive medical procedures, or even emotional turmoil such as a heated argument.
Have you been able to work with a pain management specialist regarding your back pain at all? If not, that might be something worthwhile to consider as reducing that pain could help you control your autonomic symptoms better if it's one of your triggers. The pain management community here on MedHelp is wonderful, and it may be worth posting something over there to see if they have any input as well. You can find them here:
http://www.medhelp.org/forums/Pain-Management/show/53