David this is a hard question as little amounts of pyro make a big change. Typically technical grade chemicals are in the 94 to 96% purity, so I find hard to beleive that a 2% difference in purity has made such a big difference in your developer strenght. But with pyro it could be possible. On th other hand how about your C solution, have you changed this? could you have made a mistake and mixed this stronger? Sodium carbonate is usually the culprit for things like this.
One more thing, could you have made a mistake when mixing sol A?
If you are using 1:1:1:20 then you would have oxidation very quickly in the developer. Have you seen streaks? have you seen more overall staining? examine all of this and let me know.