You're seeing the square wave on the retrace of the beam. The electron beam is supposed to be blanked during the sweep retrace (the return to the left side of the screen) but it's leaking through. Some scopes just do this if you turn the intensity way up; or there may be something wrong, possibly a bad part, posssibly an adjustment, in the intensity/blanking circuits. The low-end scopes often use a cheap pseudo-DC coupled blanking circuit that doesn't work well at high intensity.
John