On researching this phenomenon once I got home it seems this occurrence happens once or twice a year, and is attributed by some experts to the creatures having become dislodged by rough waters.
Most starfish it states usually travel alone or in small groups, but congregate in their millions if a particular area of the seabed is rich in food or to reproduce. In such situations they are very vulnerable during bad weather. Strong currents carry them to the shore and unceremoniously dump them on the beach.
Apparently due to environmental hazards and overfishing of mussels, starfish are moving closer to our shores, becoming more vulnerable to the tides. ‘Wrecks’ as they are referred to; occur mostly in the winter months.
(Primary source of reference Emma Reynolds MailOnline News 2013.)