I recall that Don Mattingly was forced to remove his pitcher due to two mound meetings in an inning. I've read articles that indicate that a limit was placed on manager-led mound meetings to speed up the game.
The MLB rule reference (8.06):
8.06 A professional league shall adopt the following rule pertaining to the visit of the manager or coach to the pitcher:
(a) This rule limits the number of trips a manager or coach may make to any one pitcher in any one inning;
(b) A second trip to the same pitcher in the same inning will cause this pitcher's automatic removal;
(c) The manager or coach is prohibited from making a second visit to the mound while the same batter is at bat, but
(d) if a pinch-hitter is substituted for this batter, the manager or coach may make a second visit to the mound, but must remove the pitcher. A manager or coach is considered to have concluded his visit to the mound when he leaves the 18-foot circle surrounding the pitcher's rubber.
Are there any historical reasons that caused this rule to be put in place?