[dwm] dwm will quit after go to console then back to x several times

Hi, list,
Pardon for my poor English, I will try my best to describe the prolem I
encounter.
Everything is good except that when I go to console (vtty 1~8 in
freebsd), then switch back to x again, many times dwm will quit, leaving
just a xorg-server, I have to use C-M-<backspace> to quit xorg-server then
startx again. Not every time dwm will quit after I'm back to x,
sometimes 1 time, and sometimes more then 10 times (i.e. go to console,
then back to X; go to console then back to x; ...).
When dwm quit, it leaves nothing warning or errors.

my OS: freebsd stable_7

-- 
Regards,
  anhnmncb
 gpg key: 44A31344