[RKBUmp]

This appears to be the rule covering your situation if you had ruled it obstruction. Still looking for references to a collision without obstruction.

12.13.1 A defensive player shall not block the base, plate or baseline without
possession of the ball or not in the immediate act of catching the ball.
EFFECT—Delayed dead ball is signaled. Obstruction is called and the
runner is declared safe. Each runner must return to the last
base legally touched at the time of the infraction. Exception: If
the runner collides flagrantly, the ball is dead, and although the
runner is declared safe on the obstruction call, she is ejected.
(Behavioral ejection)

The only reference I can find to calling the out is if the runner crashes a fielder in posession of the ball waiting to make a tag.