ISIS is really a coalition:
A. Hard core life long and total jihaders
B. Sunni militia groups tired with Iraqs Shia govt.
The "B"s, though willing to chase retreating Shia units, and willing to commit small scale war crimes, probably did not sign on to fighting US precision airstrikes and may have second thoughts about committing the large scale atrocities that the "A"s have made group policy.
Thus, hopefully, US strikes may cause the "B"s to leave.