I don't know why they said few years ago that bitcoin is bubble, but I get the picture now and I think bitcoin is becoming a bubble, but it is not yet in a bubble.
Problem is people are investing money in bitcoin just because they've heard that they can earn and they are not using it as currency to actually buy things. This is creating bubble effect and at some point it will pop.
What is bubble? Use google - http://www.investopedia.com/terms/b/bubble.asp.