The rapid price growth of the bitcoin in this 10 years made the people to think thast it was a bubble and there is a theory says that if something had huge value in short term will explode at one point so that is the reason of bubble story.But people don't understand why the price increases rapidly because of it limited quantity.
Most who had no bitcoin and had watch it grow by negligence are the ones who are mostly found calling it a bubble. The more news go around the more the demand the higher the price, volume of demand is the prime factor to the price