File names

Time for my weekly dumb question....
Recently my D70 has been swapping in and out of naming files DSCxxxx.nef and _DSCxxxx.nef It's been doing it for a while now.. and it makes things very confusing any ideas why it would do this ?
Recently my D70 has been swapping in and out of naming files DSCxxxx.nef and _DSCxxxx.nef It's been doing it for a while now.. and it makes things very confusing any ideas why it would do this ?