There are many reasons why the value of a currency might go down. Printing too much money (increasing the supply) is one of them, but it is not the only possibility. Decreasing the demand is just as effective.

Imagine that someone just found a critical flaw in the algorithms used by bitcoin. Everyone wants to sell and move to dogecoin a.s.a.p. But nobody wants to buy those bitcoins anymore! Bang, that's inflation right there.

As long as there are alternatives to bitcoin, the fact that the total number of bitcoins is limited cannot prevent inflation from happening.