The goal of a carriage style formation is to provide an 'infinite wall' that tricks a horse into wanting to run around it instead of straight through. But if your horse is well trained enough it will try to jump the wall.
One of the biggest threats to infantry deployed in a wall was actually horses tripping and crashing through the group at full speed after the rider had been killed. That big of an opening would allow the rest of the cavalry to run through and get inside the formation.
I was reading a lot of wikipedia on cavalry and infantry.