How does the SI joint/muscle pain happen?

Personally I recommend finding a good chiropractor that is into alternative medicines

Here is a site that might explain your question
Sciatic Nerve Problems (http://www.buzzle.com/articles/sciatic-nerve-problems.html)

You can try
Hylaronic acid Wal Mart sells it in the vitamin aisle
A GOOD mineral supplement Liquid is best
Mineral Rich is one brand that is good
Vitamin B complex

Essential rubbing oils
RealHeal Massage Oil: Herbal Alternative Healing Chronic Pain - Natural Healing Bruises. (http://www.enaturalremedies.com/realheal-oil-heals-bruise-quickly.htm)

See if this also helps

Take a tennis ball. Lie on your back on the floor (not the bed). Have your knees bent and your feet flat on the floor. Place the tennis ball where your back pocket would be, and GENTLY roll onto the ball. Stop each time the pain is moving toward intolerable. Stay there for 30 seconds, and then roll further onto the ball. Do this until you are completely on the ball and you are feeling absolutely no pain. Then stay there for one full minute. If you feel pain during that minute, move off a bit, take a breath, and begin again. You should be able to stay on the ball for one minute comfortably.
Exercise for Symptoms of Sciatic the Piriformis Muscle and Nerve (http://www.healthynewage.com/sciatica-pain-relief.htm)