One year back, I started having back pain. I could not sit for more than 45 minutes. I had tripped over the steps some months back and I was thinking it must be because of that.I went to a Dr and took an x-ray. The diagnosis was that it had nothing to do with the fall , an everything to do with my long working hours. The doctor also advised strengthening my core . He referred me to a physiotherapist , and after 1 month of regular therapy I could now sit for around 3 hours before light pain would start.
I felt ready to start exercising and zeroed in on yoga.
I started practicing with a group of people , led by an instructor. It was very intensive (hatha yoga) and most exercises involved stretching your arms and touching your feet.Other exercises like the bow pose, crow pose and head stand was also there.
Sometimes, while I was sitting and touching my foot, the instructor would use his body weight to push my spine down from behind so that my stomach touched my knees.
This may be the reason , but am not sure.
So, after 3 months of this , I felt better and fitter than ever before and joined another yoga studio near to my home.
Here, the poses were not as tough as the other place, but was done considerably faster.
1 month after joining there, I am bending down to touch my knees , and lightning strikes.I couldn't walk and was bedridden for a week.
The time must have been around 7 pm. I went after work and did some warm ups as usual.
Coming to the reasons, I can think two things : the instructor exerting his force on my spine and the faster pace of the exercising. This is onle conjecture though.
Right now ,after some months of physical therapy and no other exercise, I have no pain while sitting.
I certainly plan to go back to my yoga practice.
However , before I do I need to get some expert advice and some tests done.
I hope this answered all your questions!
Edit:just to be clear, the two reasons I stated are in addition to the main point I made, that some yoga poses are not safe for everyone.