Why so many teacher resignations mid-year?

Several teachers resigned from District 150 in 2012. While it seems unusual for a teacher to resign at this point in the year, The District's, January 14, 2013, HR Report shows that teacher resignations in The District continue. Teacher resignations have increased significantly under the current Superintendent.

Resignation
Fowler, Nicholas – Trewyn / Social Studies Grade 7-8 01/01/13
Harridge, Douglas – Manual / Chorus Grades 9‐12 01/11/13
Haughwout, Amy – Manual / Math Grades 9‐12 12/21/12
Rayburn, Julia – Glen Oak / Language Arts Grade 6 01/07/13
Robins, Brianna – Glen Oak / Music 12/21/12
Schmillen, Carrie – Whittier / Parental Leave 01/14/13
Sheble, Heather – Lincoln / Language Arts Grade 8 12/18/12
Terry, Dorothy – Trewyn / Grade 2 01/04/13
Wilson, Alyssa – Developmental Center / Speech Pathologist 05/30/13
Bloom, Cori – Roosevelt / Music 12/31/12
Bowlin, Shelley – PHS / Math 01/01/13
Davis, Tasha – Hines / Grade 4 11/30/12
Williams, Amanda – WCTC / English 9‐12 12/21/12

Leave of Absence
Coleman, Annette – PHS / Asst. Principal 02/26/13 – 05/22/13
Maubach, Robin – Roosevelt / Grade 1 01/07/13 – 05/30/13

Resignation Agreement
Guthrie, Brian – Day Treatment / Special Education 01/10/13

District 150 HR Report: 38 more teacher resignations

HR report of 10-09-2012

Why so many teacher resignations in September?

There appears to be an unusual number of teacher and teacher aide resignations on the Sept. 24 2012, Human Resources report shown below. Isn't this unusual for this time of year? Don't most resignations occur at the end of the school year or the week before school starts? 

From what I understand, evaluations began almost immediately after school started. In light of that, we know that some teachers are in fear of their jobs, do you think they just quit instead of being fired.

Board Report 9-24-12 APPROVED