Being in the mowing industry forever I think I've experienced every form of trailer trouble. What you have is just a common fishtail problem. Basically its caused by uneven weight distribution. When you put the mower at the back of the trailer you unloaded the weight off the tongue. A dangerous situation as you have found out, but a simple fix... move the mower toward the front and that should do it. Of the trailer is loaded just use a more equal weight distribution maybe 60/40 towards the front. Hope this helps.