What caused the change? Well, I can tell you what else happened in 1975 and 1976: Peter Seitz struck down the reserve clause, the owners lost their appeal and then locked players out of spring training for 17 days while negotiating a new player contract, Catfish Hunter signed a $3.3 million contract and a couple of dozen players became free agents at the end of 1976. Baseball evolved from a pastoral game to a business nearly overnight; the Dodgers raised their ticket prices for the first time since 1958.
So, this made me think . . . what about the Orioles? Earl Weaver is known to be a man who stuck to his guns. To the point that in the late 60s and early 70s, the Oriole players chose to ignore Earl on many occasions and play small ball (also mentioned in the same column . . . ref to Vincent's new book). See, Earl loved the long ball and understood the game pretty well. Anyway, the Orioles were a winning organization with a lot of continuity. This is not the kind of time an organization changes much about how they will do things. They didn't change their operation prior to 1976 or after. They signed no free agents in 1975 and, after the 1976 season, they let their good new free agents go: HOF Reggie Jackson and should be HOF Bobby Grich. Before 1977, they indulged in the market enough to sign Billy Smith, a not too good utility guy with random pop. Two other guys who totaled 1 IP. This winning organization never bought free agents and decided it was too risky of a venture. In the graph below, you can see that pitching use kind of changes slightly after the reserve clause is struck down, but not really. The main change comes with Joe Altobelli.
Joe Altobelli was Weaver's surprise successor with the Orioles. This was actually supposed to happened. Altobelli was groomed in the system for 12 years under the unspoken assumption he would supplant Weaver. Weaver never left, so Altobelli left for San Francisco to manage them for three years. Upon being released, he joined up with the New York Yankees as their AAA coaching and then assuming assistant coach duties with the parent club. He oversaw the transition to a 5 man rotation there as his first season was evenly split between 3 and 4 days rest. In '78, he had a 2:1 4d to 3d rest pattern. His final season, rest was overwhelmingly 4d. The '81 and '82 Yanks exclusively used the 5 man rotation. So, when Weaver left, so did the 4 man rotation.
What will be interesting now is to see how pitching changes over the next decade or so. I can see one potential change that would benefit teams. Reintroduce the fifth stater as a swing man. Within the past decade, starts with 5 days of rest varied from 30 to 45% of the starts. This situation seems like the ideal time to skip your 5th starter. The way the schedule is written these days . . . it seems like it would be a good idea to implement that. Pitchers rarely exceed 33 or 34 starts, so I do not think this is happening at the moment. Anyway, this would enable you to get more innings out of your best pitchers without being forced to start them on three days rest or make them pitch foolishly high pitch counts. It would also make your bullpen deeper for a few games around the date when the fifth starter would normally pitch.