Your wisdom teeth are pretty fukken big, so when they take them out they leave a huge hole at the back of your mouth. That hole can't just stay there, so what they want to happen is for blood clots to fill the holes in. But if you create too much suction on them, like by sucking with a straw, they might come dislodged and the holes will be back; only this time it's not really bleeding since the flesh is healing so it just leaves a massive cavity straight to your roots basically.
I forgot to mention that the holes become very easily infected with bacteria, the same ones that give you cavities, and that can cause a lot of problems in your roots and jaw