The Carbon Monoxide is ionized, meaning that an electron is stripped from the molecule, which produces a flash of blue light with a wavelength of 420 nanometres. Note how straight it is. This tail points in the opposite direction to the Sun. Incidentally, we now know the solar wind is also responsible for the aurora. Comet Hyakutake simply didn't have much dust.
Dust is blown away from the comet's nucleus by the pressure of sunlight.
After all, when you step outside on a sunny day you don't get knocked over backwards. In space, however, we don't have to deal with these Earthly effects, so the pressure of sunlight will actually become dominant. You might also recall an experiment a few years back conducted from the Russian space station, Mir, where they unfurled a sail made out of aluminized Mylar. The pressure of sunlight actually moved a small weight. Someday we may sail to the planets instead of using rockets.