While you wait for answers which might provide you with a more 'classical' insight based on more formal history and sociology, I'll give you a few pointers from a literary and pop-cultural one, which might help expand the depth of understanding of this phenomenon.

Whatever the causes, we can note that certain kinds of books which dealt with alienation from 'traditional American life' like Jack Kerouac's On the Road and JD Salinger's Catcher in the Rye became cult hits in the 1950s. Which means that there was something in the air in the post-war period which, first off, created figures like Kerouac, Salinger, Ginsberg, etc. and then additionally, caused a significant proportion of an entire generation of people to find the work of these people resonant.

It's worth noting that Kerouac and Salinger were both veterans of the Second World War who experienced the horrors of combat, having close friends die, etc. and then coming back to an America which, given what they had just gone through, made very little sense to them. Much like the Lost Generation following the First World War (Hemingway, Fitzgerald, Remarque, etc.) they found there to be something inherently fake or insincere in the world back home after having to go through the horrors of war. And since the 'proper/formal' nature of American society was inherent to it at the time, these writers tended to rebel against these values in their work.

In other words, the 'casual' lifestyle which you are referring to was inexorably linked to the countercultural tide of the 1950s--you have to remember that this was a time when Buddy Holly was basically the equivalent of punk rock as far as the establishment was concerned. The famous performance on Ed Sullivan from 1958 should be a pretty clear indication of this (Sullivan found the lyrics to 'Oh Boy!' too raunchy and asked Holly to change them, and when he refused to, Sullivan passive-aggressively introduced the band as 'Buddy Hollet and his Crickets', and also cut the power to his guitar, which is why Holly put on a really theatrical display for the guitar solo even though no sound was coming out). All this to say that in order to understand the 'deformalization of American society', you have to look at the birth and growth of postwar counterculture (which includes everything from rock'n'roll, to the Beat Generation, to the hippies, to the civil rights movement, among others).

It would be helpful if someone could append this answer with some more traditional or academic literature concerning 50s counterculture, but if you want to get a look at it from the inside, I would recommend reading the two books I reference above, as well as Howl by Allen Ginsberg and Naked Lunch by William S. Burroughs as a sort of 'beat primer', and then read some secondary literature which considers the socio-historical context of all of those books and some of their related censorship controversies. In the same vein, you could read up on the famous comedian Lenny Bruce--the 1964 obscenity trials are considered to be a landmark free-speech case in the US, though at this point you would be dipping into 1960s counterculture (which ultimately is just a continuation of the 50s counterculture you're asking about).