It's probably one of the sugar alcohols, which tend to draw water into the bowels.
As for anecdotes: Quest bars usually don't upset my system, but other protein bars (MetRX protein bar, that was a memorable time…) and a lot of "diet" foods, like Weight Watchers desserts, cause, well, lower GI upset.
A lot of "diet" bars and sugar-free protein mixes are sweetened with sugar alcohols like Xylitol or Sorbitol. Your upper GI tract can't actually digest these sugars, but there's plenty of bacteria in your lower GI tract which digest them readily and produce a large quantity of gas.
Sugar free gum has the same effect for the same reason, at least if you chew enough of it.
As for anecdotes: Quest bars usually don't upset my system, but other protein bars (MetRX protein bar, that was a memorable time…) and a lot of "diet" foods, like Weight Watchers desserts, cause, well, lower GI upset.