There are multiple schools of thought on causes of inflation, but generally I agree with late Milton Friedman that it is "everywhere and always a monetary phenomenon". Money supply expansion growing faster than GDP expansion causes inflation.
But… that was correct? It went up, then back down, due to a very unusual (almost unique, thus far) external cause.
https://www.statista.com/statistics/273418/unadjusted-monthl...
No, it wasn't correct. By "transitory" the Fed meant "no need to do anything, inflation will go down on its own". It didn't go down on it's own, and the Fed had to act, too little too late, which is now causing prolonged inflation problems.
> It went up, then back down, due to a very unusual (almost unique, thus far) external cause.
The cause wasn't external, neither unusual nor unique, it was the Fed's start of interest rate increases, precisely at the time the inflation trend reversed.