Not so much a phenomenon, just the way polarization of light works.For whatever reason (you'll have to ask B&N), the light from the display on a Nook is polarized, that is, the light waves are aligned in a single vertical plane, rather than randomly distributed though the entire 360 degrees, as is normal (non-polarized) light.
Viewing polarized light directly isn't a problem, as you have found with your Nook without sunglasses. But to view it through any form of polarizing filter, the polarization of the light source and the filter must be in alignment - i.e. with your Nook horizontal and wearing your sunglasses normally. If you turn either the Nook or the glasses through 90 degrees, the image will be blocked. This will happen whenever two polarizing sources are used together. You can verify this very simply by wearing your sunglasses and rotating a second pair in front of them in normal daylight. looking through both lenses, you will see the light vary between full brightness and pretty much total darkness as you rotate the second pair.
What use is this, you ask? Well, if you are viewing your Nook,vertically, not much! But from the experiment above, you can see that it is possible to vary the intensity of the transmitted light very precisely, which has applications in many optical fields. Besides using a polarizing filter, another way to polarize light is by reflection (which is, I guess where it comes from in the Nook) and you can see an example of this if you look at the ripples on water on a bright sunny day. The tips of the waves will show a very bright reflected light when viewed with the naked eye. Now put on your polarized sunglasses and as if by magic, the bright peaks disappear and you can see the waves clearly. Photographers use polarizing filters on their cameras for just this purpose and these filters can be rotated to eliminate the reflected glare which would otherwise ruin the shot.
The answer to your problem, if you want to read your Nook in both orientations AND wear sunglasses, is to invest in a non-polarizing pair. Or a second polarizing pair oriented 90 degrees out of sync with your current ones to read your Nook vertically - but you might look a bit silly swapping glasses, every time you rotate the Nook
Isn't science wonderful? Have fun.!