After a little (actually a lot) of research.....
About the author:
Bruce Markusen has worked at the National Baseball Hall of Fame and Museum since 1995. In September of 2000, he became the Hall of Fame's Manager of Program Presentations, after having worked as a Senior Researcher in the Hall's Library. In his various capacities at the Hall of Fame, he has written numerous articles for publication, conducted audio-visual interviews for the Hall's archives, and narrated Hall of Fame video productions.
There is no provision in the written rules that says a fielder can’t bump a runner off the bag. “There is no such rule,” says Burley. “I understand that it is commonly believed that there is, but there is not. It’s not a rule and so there is only a general practice, [but] there is no clarity. A hard slap-tag that pushes a runner off the bag (when he had just touched it) can easily be called an out.”
This is a classic example of a situation that is not covered by the Official Baseball Rules, the published rule book that is distributed each year for use in running major and minor league games.
Rather, it is an example of “practical enforcement”—a rule that is applied in general practice—in true-to-life game situations even though it is not specifically spelled out by the rules. (Now it’s also possible that such a rule is covered under the more thorough rules manual that is given to each major league umpire; this manual, which covers the rules in far more detail than the Official Baseball Rules, is not made available to the general public.) Such a ruling, which would allow the runner to remain safe after being pushed or bumped off the bag by the fielder’s body, has been applied in a practical sense for as long as I've been watching baseball (which is since the early 1970s). And that’s really the common sense approach. Otherwise, fielders would be trying to push runners off the base at every opportunity, making baseball the equivalent of bumper cars or block-and-tackle football.
So what are we to make of all this?
In general, it seems that umpires will allow a fielder the hard slap-tag, but not a body-to-body collision that forces a runner off the base. That appears to be the general application, but these situations would become much clearer if baseball’s Playing Rules Committee would include a specific provision in its published rules, so that everybody knows the deal.
Link to entire article/reference
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