see my two new reviews of research studies in employee engagement - one is in a peer-review journal and the other is available free online from an organization - The Partnership for Workplace Mental Health / American Psychiatric Association Foundation
Attridge, M. (2009). Measuring and managing employee work engagement: A review of the research and business literature. Journal of Workplace Behavioral Health: Employee Assistance Research and Practice, 24(1), 1-16. PDF available for purchase from the publisher of the journal: http://www.informaworld.com/smpp/content~db=all~content=a917319415
The main reasons given by employees were: no longer feeling challenged (cited by 33% as the main reason); no longer liking the company culture and environment (27%); feeling there’s limited growth opportunities for them in the company (26%); feeling undervalued (24%); or feeling underpaid (22%).