Sorry, I thought abour source/drain regions while I writed channel. Non-perpendicular implantation could shift S/D regions, change profile on the channel/source and channel/drain border and produce additional gradients of electric field, increasing hot carrier injection effects, etc.
The good explanation is given by Pelgrom in chapter 11.3 of his book "Analog to Digital conversion", from springer 2010 (in second edition of this book from 2013 it could be other chapter)