Germany had a similar dynamic actually speaking about proportionally less money in employees' bank accounts. And the explanation is simple and I think already mentioned, due to the reconstruction efforts after the war the economy expanded a lot and many people could raise their standards of living easily. Surely that couldn't continue until forever.
At the same time this paved the way for more hawkish politics both in the US and in Europe. IMHO this doesn't have much to do with decoupling from Gold. I think only the Money supplies M0 and MB were coupled to it but the others M1-M3 which are larger by several orders of magnitude can even be created by commercial banks.
The curve of M2 isn't changing much in direction during 1970:
https://en.wikipedia.org/wiki/Money_supply#/media/File:CPI_v...