What could have caused CFL bulbs to glow during a power outage?
This is normal for fluorescent bulbs. You should be able to see the same thing when you turn off the light fixture using a regular switch instead of dropping a tree across your power line.

Fluorescent lights generate light by an electrical discharge through the mercury vapor in the tube; the mercury emits UV light, which excites electrons in the phosphorus coating the tube.  When those electrons decay, they emit visible light.  However, there can be a delay between excitation and decay, so you'll see some light emitted for a while after the bulbs lose power.  

Variations in construction between manufacturers or even between individual bulbs mean that they'll glow for different amounts of time.