This is a very generic question.
Causes could be:
- Frustration at events
- Perceived feeling of being treated unjustly
- Feeling unrecognized or taken for granted
- Ill treatment in the office
Probably, you need to call the employee and have a heart-to-heart talk, shedding all barriers of rank/status/designation, and try to read his mind, understanding both what is said and unsaid.