Ale have a beer please

I mentioned a few days ago that Pete and I had made some nettle ale. The other night, one of the three bottles exploded making a glass/yeast/liquid mess. And on Saturday, we decided to try our well fermented ale...

The bottles had been sitting on the garage floor for a few weeks, so knowing the ale was very fizzy, I didn't move the bottle at all when I opened it. As I flicked the top of the bottle open (it had a top like Grolsch bottle), the rather potent mixture errupted like a volcany spraying about a metre and a half into the air for a good 5 seconds. There was very little left in the bottle - it tasted awful and wasn't drinkable!

Where did we go wrong?!