I realize this thread is a year old but thought I would post anyway.
I am brand spanking new here, and here mainly because my deck does the same thing. When cutting a hard left around a tree the deck swings wide right and engages the deck bracket against the PTO pulley. This was my dads mower and I think he purchased it around 2009. Not certain about that but I have the receipt for it at home. Anyway, I inherited is after he passed in 2011 and this tractor has always done this. It is a bit scary watching a shower of sparks as the PTO pulley is tearing away at the deck bracket. This tells me it is not a wear problem but as I think Brian said a design flaw. The length of mower deck from the front attachment to the rear rollers is way too long for the 9" wide attachment at the front to prevent it from swinging. The thing that stumps me is that it doesn't do it when making a hard right. Does this mower not turn as hard right as it does left?
I will double check the bolts at the rear height adjustment, and also the rollers for excessive wear but as I said it has done this since brand new. The best thing I have dreamed up is a way to attach a cable from left side frame of the tractor to the right side of the mower deck, to limit just how much it can swing right. I have not attempted this yet as I am recovering from Bi-Lateral knee replacement and not quite ready to crawl around on the garage floor.
I am also fighting mower belt issues at the moment and suspect worn bearings on the spindles and possibly the idler pulley, although the failed belts do show signs of getting into the deck bracket which will chew them up pretty quickly.
I love the concept of this Mower along with the easy removal of the deck, but the grinding pulley, destroyed belts and the Courageous to own a Kohler Courage have me wondering is it is worth the time and effort.
I will let you know what I find.