At the end of the game, the bots show which hand they were working towards. Level 2 and 3 bots have the ability to play defensively, which means that if they realize they're unlikely to win, they will break the hand they were working towards and throw tiles they think you don't need (potentially jokers) rather than risk throwing hot tiles that they think other players (including other bots) need to mahj. When this happens, the bots will indicate "(broke hand)", so you know that's what happened.
Note that in this case, looking at the bot's tiles, it probably will look like the bot was doing pretty poorly. They may have had a great hand they decided to break, and the tiles you see at the end at are the result of that.