For me, it was a growing feeling that the department I was in was falling apart. Eventually, it did.
Same with me - I had gone through a few supervisors (like four in eight months!) and I had a very bad feeling - I left marketing and ended up in R&D. Much better. Now my old group is completely gone - and my group is hiring. 
In my case it would have been going from babysitting the production of an obsolete avionics box, to designing the next generation of such boxes. I’d been doing the former for a couple of years, and had shone on the lone design related bone that was thrown my way only because others had declared the task was impossible.
So I wanted to go from mind numbingly dull work, to challenging work that I had displayed a talent for.