So what does that mean?

Rising prices -> More new investors buying into the currency -> further rising prices -> more new investors buying into the currency.

What happens when the price eventually plateau's and stabilises? Well we know that particularly Bitcoin isn't a good currency to actually use in daily transactions, so people are going to want to take their Bitcoin winnings and do something with them.

So they withdraw -> prices drop a bit -> people withdraw more -> prices drop a lot.

It's bounced back before, but eventually the bounce back won't happen. There can only be so much new money pumped into the bottom to the people in the top - it's all a zero sum game, and the rich people at the start were just paid by people who joined later. As all pyramid schemes say, 'if you start now you won't be at the bottom of the pyramid!'.