i imagine it's because over time the moisture in the bread evaporates the underside of the bread bag is colder than the rest of the bag as it is in contact with the fridge, and the water vapour condenses  at the bottom of the bag producing the water you see

I think it has to do with the heat surrounding the refrigerator.

The cooling inside the fridge is at the expense of the heat being dissipated to the exterior of the refrigerator. Depending on the model of your refrigerator, the heat is radiated either through a grill system located on the back or through the side walls of the refrigerator. In either case, the dissipating heat would lead to convection currents of relatively hot air; which rise upwards.

The rising hot air would impose a slight dessicating effect on the bread kept on the top of fridge thus drawing out moisture from it and at the same time it won't allow this moisture to condense on any of the walls in contact with air.

The moisture has no other place to settle except the surface which is not in direct contact with warm air - which is the bottom surface , the one which is closest to the fridge.

I think  we have the answer!!

Thanks for your answers.

It was simpler than I thought, I didn't recall that that part of the fridge was cool, I suppose that it was actualy warm, because it needed to disipate heat in order to cool the inside of the fridge.

But no, I checked, it was cold.

Thanks. :)