Every now and then my back goes out on me. It seems to happen at the oddest times. I'll be doing the simplest thing, like reaching for a glass of water or turning off a light. What's causing this to happen?

It's well known that once you've had back pain, you're likely to have it again. A recent study showed the spine has various loads with different motions. Muscle activity is
different from normal when lifting (or reaching) off-center for people with previous back pain.

It may be the muscles around the spine and in the trunk contract too much. The body tries to make up for a loss of stability whenever it can. Since heavy loads need more control
than the spine has, it gives all it can for less taxing activities to make up the difference.

Pain that occurs with simple daily movements is actually a sign of possible relapse. It's likely that you still need some rehab to help the muscles regain their normal coordination and pattern or contraction.

William S. Marras, PhD, et al. Spine Loading in Patients with Low Back Pain during
Asymmetric Lifting Exertions. In The Spine Journal. January/February 2004. Vol. 4.
No. 1. pp. 64-75.