I think that sometimes procrastination is a way of managing stress. When there's too much on your plate already you don't feel up to coping with something else, so you procrastinate in order to buy yourself time to calm your nervous system down.

On a good day you may be up for 'procrastination' but when you're at (or over!) the limit of what you feel like you can really cope with, then you procrastinate as a self-nurturing or maybe self-preserving mechanism.