As carcasses dry, ligaments contract and distort the body (Weber
1980). Also, dead animals are often disturbed by scavengers
and/or water currents before their remains become buried. This can
account for the contorted positions.
Some fossils do form by rapid burial, but these indicate only local
catastrophes, such as landslides of a river bank.
References:
Weber, Christopher Gregory, 1980. Common creationist attacks on geology.
Creation/Evolution issue 2, pp. 10-25.