Decades ago, people made a living from working for one company. Today, getting the itch to move on is a common occurence. Many people view this as a way to boost their career or make more money. If this type of turnover is happening frequently there could be a bigger problem. You might consider an employee retention survey to test the waters.
This type of survey is geared towards the employees. If you do administer this survey make sure you get all company members to participate. If for some reason you can't get 100% participation, try to test employees from various areas of work hours and duties.