[Moon] Power MOSFET failure

[Vladimír Petržílka]

May I ask the community please, if someone from you have an experience
with similar curious failure of PWR MOSFET (in my case MRF141 in
driver), which suddenly have been turn ON, because of 2 kOhm leakage
between drain and gate have occured? Driver has 8 years of operation.
The transistor is from RF perspective OK, because when the BIAS was
readjusted, transistor works fine again. What I made wrong? Is it just
strange transistor failure, or it can be result of some design or usage
fault? 
Thank you for any idea. 
regards
Vladimir ok1vpz
www.ok2kkw.com

PS: The driver is here: http://www.ok2kkw.com/00000104/driver/driver.htm