Monroe is a quality product.
There is no real way to answer you question for what is causing it because of your limited resource and experience to do it yourself. So unfortunately, I recommend taking it to service shop and have the suspension, chassis and the subframe carefully inspected. Not saying it is...but could be as simple as a mushroomed strut tower or even a crack in a supporting area of the suspension that was not seen originally???
There is no real way to answer you question for what is causing it because of your limited resource and experience to do it yourself. So unfortunately, I recommend taking it to service shop and have the suspension, chassis and the subframe carefully inspected. Not saying it is...but could be as simple as a mushroomed strut tower or even a crack in a supporting area of the suspension that was not seen originally???