I’m not sure why it happens exactly, but somehow, with the baby growing inside you, your body cannot break down the sugar that you take in well/fast enough, causing the diabetes. A small group of people also get diabetes permanently after gestational diabetes, which is why you should repeat the test about 6 weeks after you deliver.
It's quite common for pregnancy. Most importantly you must eat everything in moderation from the beginning when you know you are pregnant. Cut down on your sugar & carbo (especially 1 week before your OGTT). Try to drink more plain water to clear away excess sugar.