How do thrust reversers in high-bypass turbofan engines counteract initial thrust?
There are several ways to describe thrust, which amount to the same thing. Very broadly, it's described by Newton's third law: if A moves forward, B moves backward with equal momentum. On a closer level, it comes from pressure differences across the surface of a body. Even closer, you'd be looking at viscosity and velocity, laminar and turbulent flow, boundary layers, analytical equations.

But all these ways come to the same result: if one, applied correctly, predicts result X, the rest can't predict an opposite result. In the most basic view, the reverser's thrust comes from pushing air forward. As long as the ultimate result is air being accelerated in some direction, however it happens, the thrust will be in the opposite direction. 

If you want to get into the mechanics (which isn't quite necessary), the exhaust produces high pressure between the engine and the thrust reverser. This pressure acts on the thrust reverser's inner surface. The resulting backwards push is indeed greater than the fan's forward thrust.