http://mlb.mlb.com/mlb/official_info/official_rules/batter_6.jsp 6.08 The batter becomes a runner and is entitled to first base without liability to be put out (provided he advances to and touches first base) when -- (a) Four “balls” have been called by the umpire; Rule 6.08(a) Comment: A batter who is entitled to first base because of a base on balls must go to first base and touch the base before other base runners are forced to advance. This applies when bases are full and applies when a substitute runner is put into the game. If, in advancing, the base runner thinks there is a play and he slides past the base before or after touching it he may be put out by the fielder tagging him. If he fails to touch the base to which he is entitled and attempts to advance beyond that base he may be put out by tagging him or the base he missed. I think that explains what happened, if I'm reading it correctly.