The holds might be a result of some of the things you are identifying like returns and late shipping, if you have any acutal late shipping results. (comments from buyers don't count as anything unless they open cases, you don't have any neutral or negative feedbacks, but presumably feedback doesn't count against you anyway)
The place I would look is in your "performance" tab, if you go into it, are you showing as top rated?
Do you have defects or late shippings?
Defects and late shipping will for sure impact you, somewhat heavily because you sell a low volume of high value items, so it doesn't take much to give you a high relative rating, however they're supposed to offset that in these situations by requiring it to be from 5 different buyers etc before they take actual action, but I don't know what provokes the "bots" to do things like holds.
Your account isn't new, it doesn't look like you've just reinvigorated your store, so my own guess is that there's something in your performance results that is "on the edge" which is what is causing the frequent longer holds.
I don't have holds, but my account is ancient so I don't have relative experience to share in that regard.
Check out your performance results in the different domains (US, "Global" which is where Canada is) and see if you have any dings, if so that is probably what's causing it......
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