There are many possible reasons for lower back spasms such as bad posture, sprains, strains, and muscle overuse.
Lifting
Many sports like golf often cause people to experience lower back spasms because of the repetitive movement of twisting your back.
If you have weak abdominal muscles then your back is more at risk to end up with back spasms. Not getting enough exercise can lead to weakening you back muscles causes painful muscle spasms.