Was there a fairly strong north wind right before your trip that let up? It could have "pushed" and "stacked" water from the shallow upper end of the lake toward the lower end. Then when the wind stops, or turns around and blows from the south, the water would flow back to equilibrium. Only seems to happen in large shallow areas of a lake.
"Give a man a fish, and feed him for a day. Give him entitlements and you may feed him forever."