@peterdrake Overwatering, likely, which doesn't necessarily mean that you are giving too much water (ah!), it could also be that the soil is too clayish and won't drain properly.
Nitrogen deficiency wouldn't be my first guess, those top leaves are of a very deep green and those burnings at the end are not only on the more yellow ones. It may still be, though, young leaves are the first affected by lack of N.