It seems like something like this has been coming, perhaps inevitable since the weekend. At that point things settled into a perilous equilibrium. At that point it became clear that the protestors were not backing down. And the military made clear, at first implicitly and then explicitly, that they would not use force against essentially peaceful protests. For Mubarak, standing pat became untenable. So there was the dismissal of the government, the appointment of a vice president and yesterday the announcement that he would not stand for a second term.
And yet, there was a problem. Often in these cases, things move really rapidly and in pretty short order the fallen leader shows up in Saudi or Switzerland or some place like that. But it hasn't happened. Some of that must stem from Mubarak's stubbornness or pride or refusal to be cowed -- depending on your point of view. But that's also a dangerous situation because while the protestors had the power to break his rule they have not yet seemed able to end it.
It also raises a thorny problem for the US. Had Mubarak gotten to this point 4 or 5 days ago, he might still have enough international support to put this down. But now the US and and the other world powers are basically on record saying he needs to go. Even if they wanted to, they can't turn back the clock.
Into that impasse, all sorts of reckless behavior can be thrown. And now it seems that it has.