I was about 16 when I first went to the docs with a bad back. He said 'it'll get worse as you get older'.
Yes, sudden movements can cause spasms. I now have 45 years worth of experience. It depends on what is causing it but usually it is the disk between the vertebrae that deterioriates with age. You don't need a dramatic movement or have to be lifting something heavy, simply stretching to pick something up as light as a newspaper can cause the disk to touch the nerve and your legs just drop away. The pain is caused by the inflamation and, because it is so hard to find a comfortable position, you hold yourself in all sorts of subtley odd poses which just make it worse.
Eventually, the disk completely degenerates and your vertebrae fuse together and you shrink a little.
When you feel better, start taking precautions, sit upright and don't slouch, no sudden movements, keep good support on your back when driving and, oddly enough, wear good shoes and make sure the soles are not unevenly worn. Walking with a good posture helps greatly. All my bikes are adjusted so that I sit upright, like traditional sit up and beg styles.
Ibuprofen needs to be 600mg for the anti inflamatory bit to work, but you need a prescription for that. Last time I asked for a box of 400mg tablets and a box of 200mg tables they caught on and wouldn't sell them to me.