Mostly formerly because it is still celebrated only in Hungerford, Berks -- once very popular, it never really recovered from being banned by Henry VIII. The ostensible reason for the traditional festivities was to remember the massacre of some Danes by Ethelred the Unready in 1002, or possibly the death of Harthacanute in 1042, even though both those event happened in very different seasons. Any excuse for a party, eh? More at the Wiki for the curious. As for the name, hock- is a prefix for things of the second week after Easter, of origin unknown and no longer used much either.