I grew up on the Pacific coast, played in waves and body-surfed as a kid. The further the water receded after a wave crashed on the shore, the bigger the wave that would follow it. I think that if I was on the beach and saw the water pulling back at such an extreme rate, I would instinctively run like hell to get away from the beach and to the highest ground possible. Sounds like these gypsy folks had that one down.
The tribe in question was taught to run for the hill after a earth quake, as I recall lot's of freeper were posting "surfs up" when news of a big earth quake in the Indian Ocean hit FR. Not rocket science but alot of people never listen to the news and a big low tide in and of itself doesn't alway mean a 25' tsunami.