Gravitation, the attraction of all matter for all other matter. Plain and simple.
You would fall over because your overall force (centripetal vs inertia) would act outside of your point of contact with the ground- your feet. Similar to standing in a bus as it goes around a corner, you need to lean into the turn or you fall over.
In fact, the spherical shape of the Earth allows perpendicular standing as essentially you are falling over, but gravity keeps you perpendicular with the core of the Earth, when with a flat earth you would slide sideways, or as above, fall over.
Also, you have terrible attitudes, some of you (Pvt Douchebag for instance), and it doesn't give a good impression to newcomers to the site. Would I want to change my beliefs so radically and join a new community of arsewipes at the same time? No. So get a fucking grip, some of you.