Bird - on my mower the top link attatches to a U shaped piece of metal, which is free to pivot in a vertical plane as the mower raises and lowers. This U bracket is attatched to the lift arms of the mower itself. When the mower is on the ground, the open part of the U is facing the sky. As the mower is lifted to mowing height, the open part of the U bracket moves forward and down (counterclockswise if you are observing from the left side of the tractor). The normal arc of rotation of this bracket is therefore from the 12 oclock position when the mower is on the ground, to about the 7 oclock position, again, assuming you are standing on the left side of the tractor. This is therefore about 150 degrees of arc. When the mower raises too high, as when I am going up a steep hill backward, the bracket can swing past a straight line with the toplink, and past a 180 degree line if you were to draw a line along the toplink and extend it on to the U bracket. That is, the U bracket attempts to swing to the 6 oclock position. After that point, if I try to lower the mower all that happens is the U bracket tries to continue DOWN AND BACKWARDS (counterclockwise from the left side of the tractor) rather than the normal return arc of upwards and forward (clockwise from the left) as I lower the mower. That is, it tries to swing to the 5 oclock position, but can't because the lift arms are in the way. If I could draw a picture it would be crystal clear, but the problem as I see it is that the mechanics prevent the normal "counterclockwise rotation as the mower raises and clockwise rotation as the mower lowers" pattern because the line between the toplink and the U bracket goes past 180 degrees. Until I hit that 180 degree point, the linkage works beautifully.
