plugged USB keyboard (as secondary)

ukbd0 at uhub0 port 1 configuration 1 interface 0
ukbd0: CHICONY Keyboard NT68P81, rev 1.00/0.01, addr 2, iclass 3/1
wskbd1 at ukbd0
boot device: wd0
root on wd0a dumps on wd0b
IP Filter: v3.4.9 initialized.  Default = pass all, Logging = disabled
wsdisplay0: screen 1 added (80x25, vt100 emulation)

it is not attached to mux.

when i do cat /dev/wskdb1  it dumps codes for keypresses/keyreleases well.
but... after killing cat and few seconds i've got:

uhub0: port error, restarting port 1
ukbd0: at uhub0 port 1 (addr 2) disconnected
wskbd1 detached
ukbd0 detached
ukbd0 at uhub0 port 1 configuration 1 interface 0
ukbd0: CHICONY Keyboard NT68P81, rev 1.00/0.01, addr 2, iclass 3/1
wskbd1 at ukbd0

then it works well anyway but every each open/close of /dev/wskdb1 i've
got port error

what's wrong?