Plum wine gone mad! What's going on?

[Boozehound]

I've been trying to make a plum brandy. I got about 10 kg of plums and steamed them a bit to extract some juice and to sterilise them. I cooled this and added to sugar and DAP to make a wash of these parts:

10kg steamed plums
4 gallons of water + steamed plum juice
3.5kg of white sugar.
2 tsps of DAP

I added this to the lees of a previous sugar wash using EC 1118 in a 6 gallon plastic fermenter.

Then things started to go wrong.

Within an hour or so the wash was foaming so much it had pushed the cotton wool plug onto out and was dripping down the outside. It also pushed all the steamed plums out with it. I kept these and put them in the fridge. I had to put the whole thing in my bath to contain the mess it was making. I left it overnight to burn itself out. Overnight it went mad and foamed more.

Today the foaming has stopped but I've put an airlock on it and it's not bubbling. I'm not sure about hydrometer reading because I've got fruit still floating in there, but the taste still has sweetness so not all the sugar has been used.

What happened here? Why was the reaction so violent?

What should I do next? Should I add some more yeast and try and get it going again?