Today happens to be World Oceans Day, a day first designated in 2002 to honour the world's oceans and celebrate the products the ocean provides. More information here.
To this point there have been no official statements or tests conducted to clarify the cause of the crabs' mass exodus.
In related news, Tokyo Electric Power (TEPCO) this week began the construction of an underwater ice wall to staunch massive amounts of contaminated groundwater that have been flowing into the sea from the crippled Fukushima Daiichi nuclear complex. Read more here.
Whether there is a link with the dead crabs found in Phuket this morning has yet to be confirmed or dismissed.