More than 800 of the creatures were discovered on a 1,000-foot stretch of sand on Ishigaki Island in January, a report in the journal Coral Reefs reported.
A population "outbreak" was first noted in 2009 when masses of juvenile starfish were seen feeding on the island's outer coral reef, eventually completely deleting the food source in the next three years, researchers said.
"The shortage of food, corals, is a probable cause of the stranding," Go Suzuki from Japan's Fisheries Research Agency said.
An area once covered with up to 60 percent coral was reduced to 1 percent, he said.
The starfish gradually moved closer to the beach, possibly in search of more coral to feed on, researchers said, and when currents pushed them onto the beach they were too weak to return to the sea.