I know only about one explanation:

When treating about acid soils, the pH increases to a range of about 6.5-7.5 during more or less 3 weeks after inundation due to reduction of iron and manganese oxides, what leaves free ions OH- (more or less literal translation from Spanish).

In alcaline soil, the pH decreases due to increase of partial pressure and production of CO2 (carbon dioxide). It is also a literal translation.

These changes depends on initial soil pH (lower pH, the greater change), the quantity and nature of reducible components (if soils contains little iron and is rich in organic matter, the pH increase after inundation is not significant) , quantity and nature of organic matter, temperature.

I don't know current sources, there were some publications in 1950's and 1970's but I don't know, if they were digitalized:

Ponnamperuna F. N. 1955. Dynamic aspects of flooded soil and the nutrition of the rice plant. In IRRI: The mineral nutrition of rice plant, 295-328,

Sanchez P. A. 1972. Fertilizacion y manejo del nitrogeno en el cultivo del arroz tropical. Suelos ecuatoriales IV(1): 197-240.

This references I found in Spanish chapter of the book I have:

Leon L. A., Arregoces O. 1985. Quimica de los suelos inundados. In: CIAT 1985: Arroz: Investigacion y produccion,287- 305.

I took this explanation from this last mentioned book.

I know only about one explanation:

When treating about acid soils, the pH increases to a range of about 6.5-7.5 during more or less 3 weeks after inundation due to reduction of iron and manganese oxides, what leaves free ions OH- (more or less literal translation from Spanish).

In alcaline soil, the pH decreases due to increase of partial pressure and production of CO2 (carbon dioxide). It is also a literal translation.

These changes depends on initial soil pH (lower pH, the greater change), the quantity and nature of reducible components (if soils contains little iron and is rich in organic matter, the pH increase after inundation is not significant) , quantity and nature of organic matter, temperature.

I don't know current sources, there were some publications in 1950's and 1970's but I don't know, if they were digitalized:

Ponnamperuna F. N. 1955. Dynamic aspects of flooded soil and the nutrition of the rice plant. In IRRI: The mineral nutrition of rice plant, 295-328,

Sanchez P. A. 1972. Fertilizacion y manejo del nitrogeno en el cultivo del arroz tropical. Suelos ecuatoriales IV(1): 197-240.

This references I found in Spanish chapter of the book I have:

Leon L. A., Arregoces O. 1985. Quimica de los suelos inundados. In: CIAT 1985: Arroz: Investigacion y produccion,287- 305.

I took this explanation from this last mentioned book.

The values of redox potential  (RP) higher than +200 mV, which reflected aerated conditions in soil. The values in the subsoil were always lower than in the topsoil. From the chosen variables, redox potential was negatively correlated with moisture and pHKCl in the subsoil and in the topsoil negatively correlated with maximal capillary capacity and bulk density. 

The values of redox potential  (RP) higher than +200 mV, which reflected aerated conditions in soil. The values in the subsoil were always lower than in the topsoil. From the chosen variables, redox potential was negatively correlated with moisture and pHKCl in the subsoil and in the topsoil negatively correlated with maximal capillary capacity and bulk density. The positive correlation was found between redox potential and porosity.

There is no explanation, but one maybe small aspect of your dillema. During digestion organic matter in the reducing space, it tends to production NH3, that makes an alkaline reaction. Long ago I wondered, why, sometimes, pH of compost was over 9, now I know - it was wrong seasoned, not enough air. And sorry for my English.

Thats a good point to put in Ezzat . Whether soil is acid or alkaline in nature , soil pH , most of the times is buffered around neutrality. For the simple reason , soil possess maximum buffering capacity due better equlibrium between cations and anions  at neutral pH, besides much better mineral stability.

In natural environments, the oxidation reactions often release H+ ions, whereas the reduction reactions consume H+ ions. So the oxidation of Fe2+ to Fe2O3 or Mn2+ to MnO2 release H+, while the reverse reactions (reductions) consume H+, as Dr Stepien says. Similarly, the oxidation of NH4+ to NO3- releases H+ and the reverse reaction consumes H+.

Fe2O3 + 6 H+ + 2 e- = 2 Fe2+ + 3 H2O

MnO2 + 4 H+ + 2 e- = Mn2+ + 2 H2O

NO3- + 10 H+ + 8 e- = NH4+ + 3 H2O

Moreover, in rainy climates and well-drained soils leaching processes lead to acidification. In reduced soils, these processes are limited or nonexistent.

I compliment the good replies of Dr.Stepien and Dr.(Ms.)Maria Luisa for their good replies and nearly answering the question.Under flooded conditions,due to  reduced conditions, the oxidants,NO3,MnO2,Fe(OH)3,SO4 and CO2 undergo reduction , by accepting electrons and consumption of H+ ions, to form water molecules.This results in an increase in pH of acid soil.In alkaline soils,pH is controlled(lowered) by the accumulation of dissolved CO2 and organic acids.After a few weeks of flooding the soil pH attains stability and remains around 6.6-7.0.In flooded soil there is negative correlation between redox potential and soil pH as mentioned by professor Esmail.The following references will provide very good details of the subject being discussed.

Anaerobic soils by Inglet,P.W et al.2005

https://soils.ifas.ufl.edu/wetlands/publications/PDF-articles/283.Anaerobic Soils.In Encyclopedia of Soils in the Environment..pdf

Redox potential,nitrate content and pH in flooded EutricCambisols during nitrate reduction.Wlodarczyk,T.et al.

www.agriculturalJournals.cz/publicFiles/00103.pdf

CentralMetabolism(Flooded soils)MicrobeWiki https://microbewiki.kenyon.edu/index.php/Central_Metabolism_(Flooded_soils)

Anaerobic environment is usually limited by electron acceptors(oxidants) and having abundant supply of  electron donors (reductants) e.g (ammonium.iron2,Mn2.CH4,and CO2. We can see the hydrogen ion is already consumed as Dr maria mentioned.so all amjor redox reactions such as denitrification and reduction of Mn, Fe, and SULFATE INCREASE SOIL pH. The pH increase is howevr limited by the precipitation of Fe2 and Mn2 carbonates occcuring at about pH 7 and production of CO2 and organic acids from decomposing organic matter. there is also a negative correlation b/w pH and Eh and many biogeochemical  processes will be significantly altered due to such ph shifts with redox fluctuations, i think so