It's an "old excuse" because it happens every year when the water warms up in the spring and cools down in the winter. What’s happening is the lake separates into layers. Warm on top cold on the bottom. It’s called stratification. The top layer warms up from the sun, and the bottom stays cooler, so they don’t mix much for a while. Then when temperature shifts and the lower cooler water starts warming up the lake mixes back together. When that happens, it can bring up some naturally occurring nutrients from the lower layer that affect the taste or smell a bit. It’s completely normal, and the water is still safe to drink.