This is usually due to an infestation of bow mites. Bow mites are common carpet beetle larvae (dermestid beetle). They live in cool dark places, and if you haven’t opened your violin case in a while, chances are they have taken up residence – feeding happily on the horsehair. The result is, as they eat through the hair, the hairs fall out one by one almost as if they had been cut. To get rid of mites, thoroughly vacuum out your instrument case spraying it lightly with a household bug killer, after which expose it to direct sunlight which will finish the job.

The bow, of course if it is a good one, will need re-hairing – or get a new one!