We'd heard rumblings yesterday that Cohen, a former candidate for the GM job, would not be back, but the Nissim departure was slightly more unexpected. However, with Idzik's reputation as an experienced contract negotiator (one of Nissim's major roles as Mike Tannenbaum's right hand man), that perhaps makes Nissim's duties surplus to requirements. Idzik also brings with him a slightly different contract philosophy with more of a focus on upfront bonuses rather than rolling guarantees and guaranteed salaries.
Once a new GM comes in after the season, they generally wait until after the draft to start making any major changes and that is exactly what is happening here. Mike Tannenbaum might have been the one who was ultimately responsible for football ops, but Ari and Scott were the two guys who helped come up with and execute his plans -- they were his right-hand men. It's hard to blame Idzik for wanting to bring in people whom he knows and has worked with, or at the very least he himself has picked.