It likely wasn't tightened enough to begin with, as you guessed, and loosened due to normal engine vibration.

On a somewhat related note, I'm ready to state with 99% certainty that electric temperature gauges on marine engines are worse than useless. After spending years chasing after a creeping temperature issue, that probably never existed, I finally decided to try a $20 mechanical gauge. Most of my marine mechanic friends agreed that a faulty gauge was among the possibilities. Running the engine hard outside the harbor never brought the new gauge above 165°F. this was confirmed by the handheld infrared thermometer which displayed 167°F. The electric gauge would have been reading near or above the maximum of 195°F. The acid test will be a good long run of 4 hours or more some other weekend. A friend tells me he once sailed back to Florida from the Abacos with what he thought was a toasted engine when he lost oil pressure. Turned out to be a faulty sensor. I'll be adding a mechanical oil pressure gauge next spring as well.