Around 10 years ago the platforms started to open up and it got a lot easier to get a game into the marketplace. Seeing the success of the previous wave of Indie games and lower barriers to entry, people started to make a lot more indie games. Competition got more intense and finding financial success became less certain.
The worry of the "Indiepocalypse" is that as this trend continues, it will become harder and harder to make indie game development a viable, sustainable business. Which isn't suggesting that people will stop making indie games, it's suggesting that they will mostly be made at a loss and it won't be a viable business for the vast majority of developers.
The degree to which this worry has come to pass over the years is debatable.