Why are LCD panels polarized at 45 degrees?
Positioning the polarizers and alignment layer in a TN display at 45° and 135° can help improve viewing angle performance.  By doing this, the angles of best contrast are placed in the display's horizontal plane.


This has to do with the asymmetric shape the liquid crystals assume in a TN display.

From: 3M Optics 101.

While all the videos are informative (if a bit repetitious) the last one, "Optics 101 - Original Flash Animations" cuts to the chase.  I'd recommend watching from the beginning, but the answer being sought occurs around the 13 minute mark, "Section 15: Twisted Nematic Displays: TN Orientation"