It appears from the photo that there may be some "blooming" where the axles have changed color? You say you rinsed them, but I wonder if you noticed whether the axles felt oily or sticky at all beforehand?

This would be consistent with the kind of deterioration that is common with Polyvinyl Chloride (PVC), a kind of plastic that was once common in toy manufacturing. I don't know for certain that these early axles were made from PVC, but I do know that LEGO did formerly use PVC for some non-brick pieces, and I have seen similar deterioration on other PVC items. This is a known problem with Barbie dolls, for example.

PVC typically has a plasticiser added to it to make it flexible, usually dibutyl phthalate. This plasticiser gradually "leaks" out of the plastic, leaving it discolored and vulnerable to deterioration and creating gasses (like hydrogen chloride) that will attack other PVC and hasten the process. Heat and light will accelerate the breakdown, and you indicate that these were stored together in a plastic bag. That might have contributed, as the gasses and plasticiser were trapped in contact with the pieces.

Phthalates are toxic and are now regulated in the US, EU, and many other countries. LEGO no longer uses PVC except in some limited applications like the insulation for electrical wiring. LEGO was one of the first companies to begin phasing out the use of PVC, in the 1990s.

To slow the deterioration of PVC, keeping it away from heat and light will help, as will keeping it ventilated to remove the harmful gasses produced by the plasticiser.