"The most common cause for "having a loose screw" is failure to install it to proper torque. If the machine shakes, rattles, or rolls, using a properly calibrated torque wrench and getting it to the correct tightness when it's first installed is critical. "

That's what I 'assume' happened to the bolts in my mower. The manager said he was "putting it on the bench for immediate attention next morning."

I'm guessing that the tech didn't like one extra task in his busy schedule, and hurried.....and the bolts vibrated loose after a few uses. I'll never know, but that doesn't make me feel better about the whole thing.