For an answer to the first question: Yes, something did happen that compelled Dubcek and the other reformers to act in early 1968. They came to power. A voting bloc within the Central Committee rallied around three demands--the reopening of the economy, the removal of censorship and other hindrances to cultural development, and the improvement of Czech-Slovak relations--and voted Dubcek (a Slovak; remember demand #3) and Oldrich Cernik (prime minister) into power. Why did it happen then, at the end of 1967/beginning of 1968? Because each problem had reached its boiling point: Novotny had just had a showdown with the Writers Union, the economy continued to stagnate, and Dubcek had openly challenged Novotny at an October meeting of the CC regarding the need for increased investment in Slovakia, to which Novotny replied, and this is a paraphrase (I don't have the notes on me; you can read the actual transcript in Prague Spring, 1968: A National Security Archive Documents Reader) "Well if the Slovaks want to make a national issue out of this, then maybe we Czechs should do the same. Maybe we should invest all of the taxes generated in the Czech lands, which amounts to almost all of Czechoslovakia's budget, to Czech development."

You might ask, then, why Dubcek and Cernik, who were certainly devout Marxists, envisioned a completely different course for Czechoslovakia than their predecessors. I am currently writing a chapter of a Master's Thesis covering this exact topic, and I'll be glad to share relevant portions when it is complete, but for now I'll just say that Dubcek and Cernik, who were born exactly one month apart, became Marxists only after Munich (the betrayal of the West), World War Two, and Soviet Liberation (keep in mind also that Dubcek grew up in the Soviet Union and spoke fluent Russian). They became Communists under much more revolutionary circumstances than their predecessors, who gained power largely because they had been Stalinist lackeys during the interwar years and the war. The new generation had a very different, overly optimistic view of the application of Marxism in highly industrialized Czechosovakia (the "Action Program," the central document of the Prague Spring Marxists, claims upfront that Czechoslovakia was the "first" industrialized nation to implement socialism, which reflected a general belief they held that Czechoslovakia should be the grounds from which true Marxism sprang), and this helps to explain why they failed to realize that the Warsaw Pact would eventually invade Czechoslovakia.

If you really are interested in the Prague Spring, you should read Kieran Williams' The Prague Spring and Its Aftermath, as well as Dubcek's memoir Hope Dies Last. Keep in mind that the memoir was written two decades after the invasion--which makes Dubcek's continued adherence to Marx and socialism that much more interesting, in my opinion.

As for the reformers, they were all removed from their posts and monitored by the secret police until the fall of Communism, although many, notably Zdenek Mlynar, who became the face of the Prague Spring for many in the West. The secret police, who essentially worked for the Soviet Union, did not want to make martyrs out of any of the reformers, so there was not a repeat of the brutal purges of the 1950s. Note that the same thing happened to many Czech writers and artists, including the famous Milan Kundera, who moved to France in the 1970s. His most popular novel, "The Unbearable Lightness of Being," employs 1960-1970s Prague as the setting. If you do read it, however, expect a very one dimensional view of Dubcek as a coward and a traitor. In any case, this book is in some ways a first hand account of the mood of the population in the wake of the invasion, which was obviously gloomy.