Members of British Divers Marine Life Rescue and other volunteers attempted to save some of the pilot whales.
In its report, SMASS said it found that a mature female had experienced a difficult birth.
The scientists said this suggested a “possible social trigger for the stranding”.
They added: “Strong social cohesion may have then led the entire group to follow the distressed individual into shallow waters.
“Once stranded, the animals were unable to refloat themselves due to a combination of surf generated by onshore winds and the soft sand substrate on the beach.”
Long-finned pilot whales are large dolphins and described by Whale and Dolphin Conservation as having “incredibly strong bonds”.