The only way it would RTH into the middle of the lake is if the home point was manually reset during flight. OP says he didn't do that. It's not exactly something you would easily do by accident.
Some facts about RTH:
- At power up, there is no home point. Home LAT, LON, ALT are all zero.
- The home point keeps getting updated (as long as the sat count stays above 6) until you start the motors.
I've never seen any evidence to support a "rogue RTH" to a distant or old location. Not saying it isn't possible but everything I've seen suggests it's highly unlikely. Maybe if the claimed rogue RTHs all headed for the intersection of the equator and the prime meridian, we might be onto something.
Solar flares? Red herring. Seriously. That information is wrong. If there was a solar flare that was so big, it caused your Phantom to flyaway, we'd all be f***ed!