I have some Elderberry wine that I started early september. I maxed the
alcohol out on the yeast (about 18%) and it's turning into quite a nice
port style wine.

A few days ago I noticed that something large was floating on the
surface. I sulphited it and left it, but it didn't go away. So today I
racked it. The large something was left behind and looked like sediment
that had clumped together (about half of it). When I rinsed the
demijohn it dissolved under the water pressure very quickly.

Only one demijohn has done this, the other hasn't. Any ideas as to why?
Is it something simple such as gasses getting trapped and lifting it up?