I wish I had known about this before the Chicago Teachers Union suspended their strike and returned to work, but it may shed some light on the timing of the suspension. At the least, it provides a little more context for what teachers unions have to deal with on the ground.
Apparently the above anti-union ad played non-stop on television in Chicago throughout the strike. It’s the product of Education Reform Now, a group that also sometimes goes by Democrats for Education

DFER Fake Democrats for Education Privatization (click on Picture) 
IT IS A RULE: YOU CAN'T BE AGAINST PEOPLE AND FOR CORPORATIONS AND CALL YOURSELF A DEMOCRAT...EVER!