1
general::
Capsize, turn upside down, as in When they collided, the car turned turtle. This expression alludes to the helplessness of a turtle turned on its back, where its shell can no longer protect it. [First half of 1800s]
American Heritage Idioms
2
general::
Phrase(s): turn turtle
Fig. to turn upside down. (See also turn belly up.) • The sailboat turned turtle, but the sailors only got wet. • The car ran off the road and turned turtle in the ditch.
McGrawhill's American Idioms And Phrasal Verbs