It’s definitely over feeding. You should give them only water for 2-3 weeks or give them a good flush if it doesn’t stop and keep spreading. In soil, nutrients gets used up after 3 weeks approximately. When the EC of the media gets too high it turns into hypertonic condition ( and pH start to drop dangerously ), the plant start to reverse her fluids to dilute the roots zone to turn it back into a hypotonic condition to balance the fluids exchange. This is why your tips gets dried and burned !