Not sure why this would have happened after being in the ground for 4 years. However, as you know, plumerias have a relatively shallow root system. Combine that with wet soil and maybe a sudden gust of wind and it could happen. I've had plumerias that are in the ground, (just for the summer) blow over during 40 - 50 mph winds associated with hurricanes and/or tropical storms.

Sounds like you have already done all you can do by staking it back up. John