The rule book defines a list of fouls. That is not a foul, unless you get very, very creative in the unsportsmanlike conduct section.
The player gained no advantage by the cue ball falling into the pocket and it probably was not his intent. The table could be blamed -- perhaps the slate had been beveled by the pocket.
I doubt that such an event is even mentioned in the rule book, one way or the other. There are some instances when you must make a judgement call and I did. The ball had been placed on the table and was now in play. Then he knocked it into the pocket with the shaft of this cue while he was lining up to shoot.