Attrition analysis is the best way to identify the root cause for attrition and can create steps to reduce the attrition rate. Based on the industry (IT, Manufacturing, ITES and so on) the attrition rate varies. Search for attrition trends by industry or sector to understand the market level.
Reason can be known by looking your own company's attrition data. In IT industry, the termination process tracks the reason for employee's leaving and uses this to devise strategies to attract and retain the talents. The reason could be any of the below
Compensation (not better compensation as per market standard)
Work environment (not a suitable place to work)
No Growth (no career growth)
Personal (health, family or any reasons)
Work life balance (not balanced work load)
Leaders/superior (if leadership is not transparent and ineffective)
The category may extend per the industry. My suggestion is please understand completely the HR processes from recruitment to termination so that you will identify where the issue happens.
Sometimes it also happens because of poor recruitment. So each and every aspects of the HR function has to be analysed deeply to identify and fix this issue.