Kilauea has been at Orange for the past several years--the ash clouds that were generated were small, highly localized, and not really a threat to aviation. On April 30, the erupting fissure (Pu'u 'O'o) suddenly had its floor collapse, cutting off the main escape path of lava. At the same time, there was a massive surge in magma underneath the summit, which shortly thereafter shifted down the flank.
The lava lake at the summit is now a crater over 300 meters deep. This means that, in addition to rocks falling into the crater and subsequently being ejected into the air in the form of fine ash, water is also intruding into the column, which raises the distinct possibility of a high pressure steam explosion. The alert level was changed because there is now a persistent, high-level ash cloud that is traveling some distance. It's not on the same scale as Eyjafjallajökull (the volcano that shut down European airspace), but it is big enough to start wrapping around the Hawaiian islands.
The lava lake at the summit is now a crater over 300 meters deep. This means that, in addition to rocks falling into the crater and subsequently being ejected into the air in the form of fine ash, water is also intruding into the column, which raises the distinct possibility of a high pressure steam explosion. The alert level was changed because there is now a persistent, high-level ash cloud that is traveling some distance. It's not on the same scale as Eyjafjallajökull (the volcano that shut down European airspace), but it is big enough to start wrapping around the Hawaiian islands.