Boy, I'm no chemist, but I know that the high-contrast developers with sodium hydroxide in them (like Kodak D-8) are blisteringly active. Typical developing times in D-8 are 2 minutes. So my guess would be that there was some non-reacted sodium hydroxide in your homebrew sodium metaborate.

I've never used D-8, but I mixed some D-19 recently and used it, and noted that when I poured in the stop bath, there was a lot of gas evolved, it was trying to push the lid off the Nikor tank. Anything similar happen to you, showing that the developer was a strong base?