When the supply of money is fixed or increasing only slowly, deflation can feed on itself.
Investors will look at the rising price of the coins, and conclude that they're set to rise further.
So they buy more, sending the price even higher.
This goes on until the market is sated.
In the ideal outcome, the value of the currency then stabilizes at the new high level.
In the worst case, the value plunges.
This boom-bust cycle has already happened once before for Bitcoin. It hit nearly $31 in June 2011, then crashed, hitting $2 five months later.