Most cassette players have electronic photo sensors to detect when the take-up reel stops turning, then it automatically reverses. If one sensor fails, it may play in only one direction as long as the tape isn't at the end. You can try turning the tape over to see if it plays one way.
Some also have a belt that runs a sensor wheel. That belt can break or it can stretch and slip. A slipping belt usually causes intermittent reversing in the middle of the tape.