I doubt one tree is more susceptible than another....aphids are equal opportunity pests :-) Why they pick one plant over another is a bit mysterious but it is common. You see it with roses all the time.......one can be heavily infested while its neighbor remains clean.

A lot has to do with the new growth. Aphids prefer very succulent new growth and one tree may be offering more of that than another.

But aphids are rarely present in numbers to do significant damage to a tree (or even smaller shrubs and perennials) and it is usually best just to let nature and her very effective pesticides - predatory insects - take care of the problem. Or spray the tree down with a strong stream of water before you try something stronger or toxic.

I unleased a bunch of ladybugs on them so we'll see what happens. Strange that the other crabs are fine but maybe just a coincidence. Thanks!