That Chip Kelly doesn't like him?
And they just cut him when they still had plenty of time between now and the season to trade him? Before the draft? Before teams had injuries?
Occam's razor. Why did they cut him now for something that, ostensibly, they've known about for four years? Why did they rush to sign Riley Cooper and Jeremy Maclin but stall on DeSean's deal? Why wouldn't they want to pay him 30 million dollars? What changed between now and 2010?
My best guess, not based on any kind of on-record source or anything, is that Jackson and Kelly most likely had some kind of falling out -- probably one that make Kelly go to the powers that be and say "well either he goes or I go". So the front office shopped him around, realized that no one was going to give them a package they liked, and waived him.
But, to keep him from going out and getting paid, they leaked this story to the paper before they did it. That way, Jackson's going to be on "please don't be a criminal" money, rather than "dude you're really fast please come catch passes and kicks for us" money.