Fog
News indicated "thick fog" at the time of the accident.
News reels show fire department ribs navigating the lake to recover the bodies and indeed there is fog. The opposite side of the lake is barely visible (and it's a small lake) while the water is absolutely still. It's also reported rescue helicopters could not intervene due to the low-vis.
Several witnesses indicated the ship "spinning on itself" prior to hitting the lake, and a loud noise on impact. Then screams for help from the occupants were reported by one witness standing on land close to the point of impact, followed by silence (when they likely drowned).
Indeed it's possible pilot entered fog/cloud soon after takeoff, became spacially disoriented and LTE'd into the lake.
At 37 yrs old and with experience in aerial work including fire he wasn't a novice. He was freelancing for the entrepreneur owning the ship as the latter was "in between" pilots.