I got hit with this for the first time a couple weeks ago. I mix up a 5L XTOL kit and put it into 5 1L datatainers.
First, if I'm not mistaken, Datatainer bottles are made from HDPE plastic. My understanding is that this isn't the best substance for a developer container. Ryuji Suzuki has
a Web page that covers this topic. I'd therefore recommend you switch to PET plastic, or better yet, glass. This will provide a better seal against air. Of course, this doesn't really explain the pattern of your failure if you used identical bottles but only one went bad....
Second, I use a hand vacuum pump to reduce the air pressure in my (glass) bottles. I've noticed that some lids simply don't produce good seals; I can pump away but I hear air leaking back in or, worse, the seal is so bad that the lid never gets "sucked down" at all. Something like this could account for your pattern, albeit without the vacuum pump; if your bottle and/or lid had some crud on it or if the seal was old, it might just not have sealed properly. This could have let enough air in to cause premature oxidation.
More generally, although five months is within Kodak's stated XTOL lifetime (6 months) and well within the practical limits reported by others (a year or more), you might want to consider using
Mytol or some other mix-it-yourself developer so that you can mix up smaller batches. Although I've never used Mytol myself, it's claimed to produce results that are very similar to those of XTOL, so switching to it shouldn't require much adjustment in your photographic practices. You will, though, need to buy a few raw ingredients and a suitable scale. OTOH, I don't know how Mytol's shelf life compares to that of XTOL; smaller batch size might not gain you anything if the shelf life is shorter.