The speculation for the decision according to the media seems to be that the team was afraid if they kept Ersson and waived Sandström to leave him in the AHL that they'd lose him on the waiver wire, and by virtue of still being on a Entry Level Contract(ELC) and therefore waivers exempt, Ersson was a victim of the numbers game.
Some have also speculated that after Ersson's injury filled rookie campaign last year the team may want him to get quite a bit more playing time in the minors than he would have gotten on the main roster as Carter Hart's backup. Since Hart returned from his concussion a couple of weeks ago Ersson only started in 2 games and played in 3 total(came in to relieve Hart in the 6-0 Bruins drubbing.)
It's abundentally obvious Ersson was a victim of the numbers game, and the future in the Flyers crease is bright. The only true question is which tandem does the Flyers want and how soon do they want to get there?