WD Harrow issues

My neighbor, who just had bypass surgery, called this evening and asked if I would disk his garden before it rained tonight.  I gladly ran out, jumped onthe WD, and hooked up the harrow.  Then I stopped by the shop to lengthen the tongue, so the WD would pick the back end of the harrow up off the ground.  I went down the road to his garden, which is tiny and has no turning room at the end for a WD and an 8' harrow.  The harrow wanted to pull to the right really bad on every pass, so much so that it unhooked the left snap coupler chain every time I put the disks in the dirt.  After I finished and stopped at the back door for a chat, I headed on across the yard towards home.  The leveling or beaming bolt in the center of the tongue broke, dropping the rear gangs to the ground.  I had to unhook it and leave it in his yard to keep from dragging the disk onthe road.  I guess I'll go piece it back together this weekend and bring it home.

So, here's the question - what makes a harrow pull to one side or the other?  The thing it pretty well worn out, and has four broken discks on the front gangs.

Thanks,

Adam