Why does a vegetable soup turn sour?
A lot of types of bacteria (and sometimes other microbes) produce waste products that can taste "sour."  And soup/stock is a good growth medium for microbes, which is why most food safety organizations recommend only keeping soup for 3-4 days in the fridge.  Six days is past the point where the soup is still likely to be "fresh"; the sour taste is most likely spoilage of some sort.

As for the question of why the airtight container was different from the open one: it could be that the open container absorbed more microbes that were floating around in the air, or it could be that the open container allowed more oxygen for certain types of microbes to grow better, or both.

Bottom line: when soup turns "sour" unexpectedly, it's probably time to throw it out.