Max Fisher, writing at the Washington Post, probably gives the most succinct explanation:
... Here are three reasons the protests came back.
First, President Yanukovych signed a new law limiting basic freedoms.
Nothing gets activists back out in the street like signing a law that imposes strict limits on the press, on peaceful assemblies, on Internet use and even on speech itself. ...
Second, Yanukovych's government is seen as drawing closer to Russia.
... The political crisis was in many ways over whether Ukraine would lean more toward Moscow or toward Europe – a dilemma it's been facing, and to some degree putting off, for its 23 years of independence. ...
Third, the core problems driving the protests are all still there.
Ukraine's politics have long been divided into two major factions by the country's demographics. What's happening right now is in many ways a product of that division, which has never really been reconciled. Just about every Ukrainian government since independence has been seen as representing one "side" of this divide, with the other hating him or her as a perceived foreign pawn. That's exacerbated by political corruption and by the fact that Ukraine's troubled economy does indeed make it reliant on outside countries. Today, Ukraine is still demographically divided, its government is still troubled by corruption and its economy is still in bad shape. As long as those things are all true, public unrest is likely to continue.
One big caveat is that, while all of these factors are problems, not all of them are necessarily unpopular with all Ukrainians. After two months of protests, many Ukrainians are tired of the public demonstrations and would like to see a return to normalcy – something that also happened in Egypt after the first few months of protests wore out their welcome. Putin is not exactly beloved in Ukraine, but lots of Ukrainians speak Russian and a smaller portion are themselves ethnic Russian, so the country is not exactly the mass of Westward-facing Russophobes that it is sometimes portrayed to be.This is complicated by the fact that some of the more violent protesters--the "Right Sector" movement--stand in the middle, neither wanting closer ties to Russia, nor Europe. Other protesters (or possibly the same) are viewed as thugs hired by the government to provoke the more radical protesters or drive off the more moderate protesters.
Russia has, in the meantime, warned that the situation in Kiev is spinning out of control, while blaming EU politicians for encouraging the protesters.