Its neither overfeeding or that "this just happens." You have good temperature due to the flat surface of the leaves (no upward curling edges), but you DO however have a problem with your pH. When your pH takes a little hit on levels, it affects the leaves casing them to twist and turn. If your plant is mature enough, you can either cut the tips off, or remove the leaf completely. The spots left on the tips or around the leaves are from NUTRIENT LOCKUP caused by your pH. Keep it at six, and check it every day. If the problem continues, just flush your system out with no-nutrated water with a pH at 5.5. I work in a Hydro store, I see this all the time.