Originally Posted by MadCityCyclist

No, it's simple physics. If you brake the front wheel more than the rear, the rear wheel moves faster than the front wheel and try to move around it, especially if there is more weight on the back. It's like a car braking on ice and then fishtailing, or a rear wheel drive car going up a hill on ice and stepping on the gas, which results in fishtailing. Trust me on this, I'm an engineer. If you know anyone who is a professional truck driver they will tell you basically the same thing. With the big trucks, they cannot even maintain the same speed going up a hill on ice because they will fishtail (although moving at the same speed, power is applied to the rear wheels going up a hill to maintain a constant speed, and the power that the rear wheels have but the front wheels don't have means that the rear wheels try to move in front of the front wheels). On ice, big trucks have to use the momentum of the downhill run to carry them up the next hill so they don't have to step on the gas. It's basically the same principle with stopping a cargo bike, in less dire circumstances of course. The rear wheel can't have more energy than the front wheel or it will find a way to move ahead of the front wheel, which isn't good.

In the situation described in your owner's manual, I don't think they were saying the Mundo would flip over your head if that is what you were thinking. But there is a good chance that it will flip to your left or right side in the right set of circumstances. In less extreme circumstances you will likely experience this as the bike doing a "shimmy" from side to side when you stop.