Why was there a (lay? parish-level? episcopal?) movement of radical liturgical reform, did this really pre-date the council, or was it spun up during the council's session? Did they think they were "reading between the lines"? Were they right? Were most council theologians surprised at how their writings were interpreted? How uniform was the liturgical overturn across different national and linguistic ecclesiastical communities?
I did in the past a comment in which i explained something about the Liturgical Movement and the liturgical reforms.
In fact, the liturgical reforms started before the council both as something did in concordance with the Vatican (for example the authorization to dialogued Mass in the '20) and as something 'illegal' (as the proliferation of versum populum Mass before the Council).
There was many movements between catholic liturgist and theologians to reform the Mass and all the liturgy: the discover of the so-called 'Apostolic Tradition of St Hyppolitus of Rome' gave suggestions for a 'pure liturgy of the first times' which we had to restore; the disvover of the plurality of anaphoras of eastern Churches and their theology about the thanksgiving and the work of Holy Ghost gave discredit to the old Roman Canon, which was seen as something of deficient; the theories about the versum populum as the primitive orientation of the celebrant in the Mass etc old these storiographic theories about intellectuals formed the ideological basis for the reforms.
If you read books as 'Das Eucharistische Hochgebet' of Jungmann (1957) you could see the demands of the reform in a very explicit way, for example in the critic of the Roman Canon as a corruption of the primitive form of the eucharistic prayer, or in the argument for the versum populum celebration as a more intimate form of celebration that is more adherent to the form of banquet, or in the de facto argumentation of the inutility of the old offertory. Also you could easily find the argumentation of Vagaggini about the limits of the Roman Canon or about the inclusion of the words 'mysterium fidei' in the 'words of consecration'.
Sacrosanctum Concilium was seen as endorsement by the new wave of liturgists, which were yet present in the papal entourage and collaborated to the autorship of this document, which opened the road to the changes that this group of people asked and they were applying locally. Adrien Nocent, a progressive liturgy that taught at the Anselmianum in Rome said to his students after the approvation of SC 'the bishop do not know what they approved'. even the 'traditionalists' did not understood the deeper meaning of SC, that both said ' there must be no innovations unless the good of the Church genuinely and certainly requires them' (SC 23) and 'The liturgical books are to be revised as soon as possible' (SC 25): the first affermation tranquillized the conservative, the second opened the road to rapid changes.
and so the liturgical reform happened. the positivity or negativity of the results isnow matter of discussion