unexpected increment for evtlog.bak(NetServer LC 3)

[fomalus]

We use HP NetServer LC 3 as our Novell Netware 5.X server.Recently we encountered a strange situation that the file HPSERVER\evtlog.bak under SYS volume unnormally increase to big size(close to 400MB) and used up almost all the free space in SYS volume.The file couldn't be removed or modified unless we rebooted server.We noticed that the system seems automatically backup file evtlog.dat which is in the same folder and append to file evtlog.bak,but the frequency is not regular.So the file evtlog.bak is increasing 25KB each time(the fixed size of evtlog.dat) and using up free space in SYS volume.Kindly please anyone can indicate the solution or hint to survey.
p.s.:attachment evtlog.bak for reference(it's hexadecimal,I don't know how to read)