I've just been playing with some treatments for an undercarbonated batch in bottles. The beer is slightly carbonated, it bubbles up a little when poured, just builds no head & dissipates instantly.
I thought I'd try opening a couple of bottles & adding some carbonation drops, re-seal & see what happens after a week or two. I put 1 drop each in 2 bottles and 2 drops each in another 2 bottles.
When the sugar was added to the beer, it fizzed up. The 1-drop bottles bubbled a little but I was able to get the cap on quickly enough, however the 2-drop bottles bubbled quite viciously, much like with the Diet Coke & Mentos trick.
What could possibly have caused this effect? Obviously it only occurs after conditioning has started, otherwise bottling day would be a huge mess.