Grapes do make you poop because the grapes are pretty high in fiber.
The grapes contain most of the fiber in the skin of the grape but also in the rest of the grapes as well.
When you eat foods such as grapes with a lot of fiber then you can get a bowel movement soon after as well.
Also if you eat too many grapes you may get a stomach ache, stomach cramping, stomach bloating and even possibly diarrhea.
I once ate 3 bowls of grapes in one day because I love grapes and that was a mistake because the next day I was on the toilet with diarrhea most of the day.
The same day I had bloating and stomach cramps but the next day I got diarrhea.
But eating a bowl of grapes or two bowls of grapes per day or every other day is healthy and although you still will poop after eating grapes you shouldn't get diarrhea from that amount of grapes.