So is it sunlight, moisture, temperature, or a combination of the above?
I would say all of the above + kind of plastic used for the bubble.
For example i you look at the star wars
Vintage line, most Rotj and Potf figures will have yellowed bubbles, Whilst SW cards mostly Will have clear bubbles. Also tri logos have clear bubbles most off the time, but these are more prone to denting, because of thinner plastic i believe.
I think you can compare MOTU with Rotj and Potf bubbles and these are just more prone to yellowing.