Washer motor coupler fails - again

I have a Kenmore washer, about 10 years old. I had to replace the motor coupler (for the first time) about 5 years ago. Had to do the same about 2 years ago, and lectured the wife about overloading the washer.
Well, about a month ago, I once again had to replace the coupler. (Maybe she forgot the lecture). Now, just weeks after replacing, it has failed again. This time she assures me that she has done nothing but a medium load since the last replacement a few weeks ago. (I believe her.)
Any ideas why the motor coupler would fail if the wash load isn't that large?
Ken
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Little to do with load size. lot to do with alignment. Perhaps some other part is wearing and causing it to be out of line when operating.
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