Ignorance, malice and boredom
I've known management who would dearly love to have been able to spy on off sick staff. They hear exaggerated or even completely false stories and its like an itch that they're not supposed to scratch.
My experience involved a popular colleague who was off on a stress related issue. While off she took up running (exercise is good for stress) and around the same time dropped several colleagues from her Facebook profile.
Within a short period of time rumours started flowing about the exact nature of her illness, why her FB profile was less open and how genuine her illness could be when people had seen her out running and playing sport.
When she returned to work it all subsided, but the speed with which she went from valued colleague to slacker and back to valued colleague was a wonder to behold.