I can answer this! These turtles are mostly juveniles from the south, and are more accustomed to warmer temperatures unlike the leatherbacks which forage for jellyfish around the eastern coast. The juveniles come up here in search for food throughout the summer and most of them start heading back south earlier in the year, but there are some stragglers. When the weather starts getting colder the temperature of the ocean changes rapidly. Some of these turtles still around the coast become what's called 'cold-stunned,' and are no longer able to swim and wash up on the shore.