First of all, I've never seen anyone in 25 years other than a pitcher balk.
That said, I looked it up to, and here's what I found.
There are 15 ways to balk.
1. switches his pitching stance from the windup position to the set position (or vice versa)
without properly disengaging the rubber;
2. when going from the stretch to the set position, fails to pitch;
3. throws from the rubber to a base without stepping toward
(gaining distance in the direction of) that base;
4. throws from the rubber to a base where there is no runner and no possibility of a play;
5. steps or feints from the rubber to first base without completing the throw;
6. pitches a quick return pitch, that is, intending to catch the batter off-guard;
7. pitches or mimics a part of his pitching motion while not in contact with the rubber;
8. drops the ball while on the rubber;
9. after a feint or throw to a base from the rubber, fails to disengage
the rubber before reengaging and pitching;
10. after beginning to pitch, interrupts his pitching motion;
11. begins to pitch while the catcher is out of the catcher's box when giving an
intentional walk;
12. while pitching, removes his pivot foot from the pitching rubber, except to pivot;
13. inordinately delays the game;
14. pitches while facing away from the batter;
15. after bringing his hands together on the rubber, separates them except in making a
pitch or a throw;
16. stands on or astride the rubber without the ball, or mimics a pitch without the ball
All of these pertain to the pitcher. However, and this is a little gray, I suppose anyone in the field could "inordinately delay the game". If the third baseman was playing in foul territory- and I have no idea why they would in the first place- but anyway. But, I guess if the umpire feels like a player is delaying the game, he can call a balk. If I was a manager, I would be pretty upset especially since the rules are a little vague here. I wonder if the official scorer charged the balk to the pitcher or not. And I could see being in foul territory being technically not in the field of play.
Personally, if this is what indeed happened, I'm kind of surprised that the umpire just didn't tell the third baseman to get in fair territory. Sounds like some over-officiating to me.
Oh, these are MLB rules by the way.
