Why the Oxford movement happened
"All this is certainly very remarkable at first sight, considering the long time that these views [i.e. the views in the Tracts - RJS] have been withdrawn from our public teaching, and how gradually and certainly they seemed to be tending toward utter extinction. These very circumstances, however, have been, under God's good providence, the chief causes of their revival. The truth is, that while our Church is bound as she is to her present Prayer Book, Services, and Homilies, there must ever be a point beyond which she cannot fall away from her professed principles without exciting the scruples and alarms of tender consciences. However silently and determinately the change may go on, this salutary check will be felt at last, and prevent matters from progressing further, at any rate within the Church. And when this takes place, and men at length begin to reflect on their existing state of belief, and turn back to survey that view of religion from which they have drifted, then the very novelty of it, and (as many an opponent of it will even confess) the touching beauty, loftiness of idea, and earnestness of character which it evidences or requires, take possession of their minds, and they proceed to advocate from affection what they took up as a duty. "
and again:
It was impossible for serious-minded men, ever so little versed in Antiquity, and in the history of the Reformation, not to see that, for a century and more past, primitive truth had either been forgotten, or looked down upon, and our own engagements to it tacitly loosened. Indeed, opinions which were openly acquiesced in by free-thinkers, and noticed with satisfaction by the world's philosophers and historians, could not but excite strange impressions in the minds of true Churchmen, partly melancholy, partly by way of contrast, leading them to look forward into the future, and to anticipate change and improvement in the public mind.
As a more proximate cause, Newman points to the political climate, where the government was acting against the Church:
[...] the plain tokens, which have appeared of late on the part of our civil governors, of an intention to withdraw the protection which Protestant England has as yet ever extended to the Church. In the defection which threatens her from this quarter, her members naturally look about for other means of sustaining her present hold upon the popular mind; and, being deprived of "the arm of flesh," are thankful to find that they have in their armoury spiritual weapons, long disused indeed, but, through God's mercy, not forfeited, and of untold efficacy.
State of Religious Parties
[...] the foundation of the English Church lies very deep, while the views and principles of a number of her children are very shallow
British Critic
[Roger]