Why do we observe band when recording CRT monitors or Flouroscent lamp via video recorder?
This effect is most obvious in light generators that brighten and dim cyclicly over time, as in an AC current powered photon generator (bulb), or the phosphors on an old CRT periodically hit by a refresh beam.

In some ways this is similar to audio beating between two different but close audio frequency signals or pitched notes.  If the frequencies are slightly different (including the fundamental of one to a strong harmonic of another), then the phase will drift in and out over time.  In audio, this creates changing constructive or destructive interference.  In video recording this might capture the brightest portion of the beam or lamp when in phase, or not capture it when out of phase, or a capture over time while the phase is continually changing in between, resulting in rolling of the brightest band.

Note that the NTSC frame refresh rate is 29.97 Hz, which will beat against half an AC line frequency of 30 Hz.