Why is there sediments in my bottles?

[njbusnet]

I have many bottles of reds on my rack and recently i've noticed that all of the opened bottles have a large amount of dark sediments forming in the bottoms of each bottle. I opened these bottles several months ago (to taste them) and I recorked them. My question is 'what causes the sediments to form?' and "is it okay for me to drink it?'. Also, i've noticed a slight change in the taste of all the previously opened bottles. Is this normal?