In general, there are many variables. The emulsion releases bromides which can act as restrainers, and other compounds (halides, silver, dyes etc). Different developing agents have differing sensitivities to bromides, and sensitivity depends on pH. Local pH can be affected by development by-products which tend to be acidic, so buffering can come into play. Oxidized forms of some developing agents can be more or less active than the initial compound, and the effect of sulfite on these oxidation products can either increase or decrease the rate of development. Diffusion in different directions within the emulsion. Aerial oxidation. Convection currents. These variables are interrelated.

Also, these things are dependent on the amount of development taking place locally. More bromides and reaction products build up where there was more exposure and more silver is being developed. So the actual image being developed can have an important effect depending on where the areas of high and low exposure are relative to eachother.

There is also the risk of unevenness resulting from non-uniform wetting, diffusion etc. at the very beginning of development. This is one of the reasons why a pre-soak is usually recommended with stand methods to help promote uniformity at the beginning of development.

Put it all together and it becomes obvious why a technique like stand development requires experimentation and practice. With some combinations a full stand procedure may work fine. With others, some amount of minimal agitation may be required.