
To summarize what happened: "KABLOOM!"
The Excelsior Altbier got bottled too early. It continued to ferment once it was in the bottle, making little gas bubbles that had nowhere to go.
Before it was bottled, it fermented for a little over six weeks, which is normally very reasonable. Clearly, it needed longer because it was colder over the winter. I have moved the non-exploded bottles to the refrigerator. Hopefully, that will slow any further yeasty activity.
We'll have to drink those quick just to be on the safe side.