Car ignition won't crank at all. Security word lights up on panel. Short time later, the car starts. Why?

I cranked my car ignition and it wouldn't start. The lights and radio still worked. My mechanic said that for an unknown reason, the Security feature of the car ('95 Buick LeSabre) was activated, and this kept car from starting. He said Security resets after a short time, and that's why I was able to restart the car. He said that he couldn't fix the problem unless he saw it happening right in front of him, that's there was no way to check otherwise. Later that day, when I shifted in the seat while making a turn (I was wearing seatbelt), the Seatbelt chime sounded, and then shut off maybe 20 secs later). This may be related to problem. Also, any chance Security could shut off the engine while I was driving (which would of course be like removing key from ignition)? Thank you.