What year and how many hours? My 2006 DK45 developed shift fork issues while still under warranty. I've also seen other postings with similar problems. In my case, it was an matter of popping out of reverse, so I don't know if this could be the same (shift fork) root issue. Although this is a small, relatively inexpensive part, the repair has a whopping labor charge, so I hope it is something simpler in your case. Maybe somebody else on TBN will check in with a more definitive (and cheaper!) answer for you.
Meanwhile, don't back into any corners while you are mowing in reverse.