I don't think that the water pre soak starts the development process at all. If it did, I would not want to drink the water. What the water accomplishes is that it softens the emulsion so that the developer then can begin to develop the film immediately without the initial wait to soften the emulsion and then beginning the development process (as in the case of not using a water presoak). Some developing agents are quite active and the effect of combining these with compounds such as sodium carbonate or even in some cases sodium hydroxide makes the combination synergistic. While the development takes place consistantly (provided proper agitation is occurring) over the whole emulsion layer, the effect is proportional depending on the amount of light exposure the silver halide layer has received. The highlights in the scene receive more light exposure, this translates to greater negative density in those regions. This is what you observed in your negatives, the beginning of density representation of the high scene brightnesses. This is a perfect example (even though extreme) of what is accomplished by varying development time in the interest of increasing or decreasing contrast for Zone system procedures.