Usually, almost never, extremely rare, etc are not excluding any unusual event. AFAIK, there was an unusual freezing event in North America this winter. But of course, this event might not touched particular location of these ferns and there could be completely another reason behind this browning of ferns. It just looks similar to frost bite that I have seen here, where extreme cold is maybe more frequent. In case of pest attack, there is also possible, that it was caused by some other damage/stress factor.