Why is this satellite going to fall from the sky? And how can they predict the day so accurately ?

Asked by Buttonstc (27605) September 23rd, 2011 from iPhone

Maybe I don’t have a good enough understanding of how the Earth’s gravitational field works.

In my understanding, that’s what holds them up there in orbit without them floating off into outer space, right?

So, if all these satellites are sitting up there orbiting around the Earth day after day and year after year, what changes?

Why/how does one suddenly “decide” it’s had enough of this?

Yes, I realize that inanimate objects really don’t decide anything, but I just couldn’t think of a better way to phrase it. Indulge me, please :)

Presumably gravity stays the same and it didn’t go on a diet to change it’s weight, so?

So, how come? And why now instead of 2 weeks ago or 6 months from now?

Obviously I wasn’t a Science major but I really am seriously wondering about this and feeling a bit dense about it.