Runner forced off of legally attained base

[PandaBear]

Looking for specific rule citation (Fed or NCAA) for the situation where a player has legal possession of a base, but is forced off/out of contact with the base during live ball play by a defensive player.

Specific situation was a baserunner returning to base on a pickoff play, in which the runner successfully retouched, and then the fielder, in possession of the ball (so obstruction is not an issue), lost balance and fell onto the baserunner who had slid into the base. As a result, the baserunner was psychically forced off the base, and while off base, the fielder, who had possession of the ball in glove, touched the baserunner with gloved hand, in what would be a legal tag.

But it could apply in a number of other situations where contact occurs by a fielder in possession of the ball, and not something you could reasonably consider malicious contact (like shoving a base runner off the base so you could then tag them out while off base).

The "Make final decision on points not covered by the rules" provision doesn't seem like it should be the only basis for correcting a situation like this, but unless I'm suffering a case of "Monday", I'm not locating more specific rules reference.