It was both the Yankees winning at a ridiculous pace the last 2 months and the Sox being on fumes because Zimmer played the regulars every day all year.
After the All-Star break the Yankees went 54-25 (.684). In August and September, they went 41-16 (.719). If they played at a mere .700 pace, game 163 never happens.
As I said, the idiot Zimmer played the regulars to the point of exhaustion:
Rice played all 163 games that year. Fine, he was young, strong and played both LF and DH, he could handle it.
Burleson played in 145 of the 147 games he was available for. He was on the DL for a couple weeks in July.
Remy played in 148 games, and most of the ones he missed were in early September when he was banged up a little (no need for a DL stint at that time of year)
What Zimmer did to Fisk was criminal. Fisk played in 157 games. He caught in 154 of those, starting 150 of them. This was after the 1977 season where he played almost as much.
Zimmer kept Hobson in the line-up all year, even while Butch was rearranging bone chips in his elbow between pitches.
The biggest culprit in the collapse was Zimmer.