Why was this actor forced to do this film?

Roy Scheider played Chief Brody in Jaws (1975)

Later I believe he was cast in a film, may have been The Deer Hunter (1978), anyway he dropped out of the film for some reason.

He then seemed to be forced to do Jaws 2 (1978)

Why was he forced? Was it because he dropped out of the film he agreed to do?
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