Metabolites and toxins can be stored in fat cells for months, years and even decades.
Jon has passed 7 previous tests since DC 2. Could his weight cut for UFC 232 released old, trace metabolites enough to show up on a drug test? The timing makes sense since his December 9 failure is probably around the time when he really starts taking his diet seriously and burns a few pounds of fat, which could possibly reintroduce the metabolites into his urine and bloodstream. I think it's a completely plausible scenario. This would also be indicative that he most likely used steroids in the past, possibly even during his college years.
The other plausible scenario is that Jones has been using PEDs that are undetectable by current testing standards but they have been contaminated with other PEDs made in the same facility. Due to poor cleaning procedures, minute traces of the unwanted steroids end up in the stuff he was buying. Sure, you're only seeing the trace elements of the testable metabolites but it could be like seeing the amount of iceberg above the water surface. There's a whole lot that's not seen.
What I don't buy is that the accidentally ingested tiniest trace metabolites from 17 months ago just show up again, except even lower this time. Instead, I like the weight cut theory.