This tip damage could be as simple as a branch, dog or some other physical brush of the tender tips that caused the browning. It looks like the tips have been broken or are bent. The only other thought is that a mole or vole damaged the roots when the fronds were still young and unfurled. Then when the leaves opened the tip growth was sacrificed in order to for the roots to support the lower leaf growth. Your sword fern looks otherwise healthy so I would not worry about it.  Keep growing, Marianne Binetti