Oxygenated tap water is rich in transition metals including Fe and Mn ions. Citric is a source of H+ and a good chelate and can drive a redox reaction in the presence of oxygen and H+ proceeding as follows:
4 Fe(2+)/Mn(2+) + O2 + 2 H+ --> 4 Fe(3+)/Mn(3+) + 2 OH-
There is also a likely metal redox couple equilibrium(s) that can be effective in recycling ions (to continue the reaction, as occurs in natural waters) in the presence of citrate acting a chelate for, say, ferric:
Fe(2+) + Mn(3+) = Fe(3+) + Mn(2+)
Cu(+) + Fe(3+) = Cu(2+) + Fe(2+)
In the presence of lab or sunlight, some further recycling of metal ions is possible.
Reference: See my comments and sources cited here.
The reaction with oxygen is electrochemical in nature with an inception period and proceeds with time.
Note: H+ is consumed, so the pH would be expected to rise.
Here is a further electrochemistry reference, in particular, Table 2, where the first listed half-cell reaction corresponds to the above after adding two H+ to both sides (creating water as a product). However, as this reaction can produce basic salts, as I have discussed previously, I believe it is more informative. Further, I do prefer my rendition, as per my prior link supplied above, it could also display a possible radical chemistry underpinning to the underlying half-cell reaction mechanics.