Uhm I don't really get why so many people are confused about it. Sugar simply got so scared of Usopp's face when he swallowed the grape, she lost consciousness, especially because she simply thought it was just poisonous and didn't expect something like that.
It's the same principle as how people claim that, when you have a hiccough that doesn't want to go away, you can lose it if someone really scares the hell out of you.
If you search on YT for vids of people who ate the hottest pepper in the world, look at how they react. I think even people died of eating it. It's the same thing here. Usopp's reaction was so unexpected, it scared the living hell out of her. Not to mention Usopp always has been quite good at pulling weird faces and having messed up reactions.
