I haven't read the article you've linked, but I am *certainly* familiar with the idea that if Marxism (or some variant thereof) suddenly became so popular among the young Catholic clergy in the 1960s, it could only have been caused by malevolent infiltrators.
The fact is, however, that Marx's theories have a *tremendous* appeal to young (and not-so-young), idealistic people who believe human society ought to be a whole lot more equitable than it is. Often, such people start out with a starry-eyed notion of the Church having *always* served as the champion of the poor and downtrodden, then undergo a rude awakening when they learn just how many times and places in history this *hasn't* been the case.
When someone reaches this point of spiritual crisis (note that they usually haven't had much by way of real spiritual formation), they become disenchanted with the Church "as it is," and start looking for a way of reforming the Church so that it *will* become a major factor in the reform of society. A person in this state is drawn almost irresistibly to the idea of creating a "new" social order--which usually involves some form of "enforced equality" (in other words, some form of *Marxism*.)
The real problem, IMHO, is that we haven't been raised to look upon life with a proper Christian sense of *mystery*, which means we look at the world around us, at our own lives and at the Gospel itself through a very distorted "lens." Through this lens, we look at all the New Testament teachings regarding the poor, and either see them as "spiritual" (hence, a "soft" obligation, with little or no practical demand on me), or as "practical" (hence, a "hard" obligation to take concrete, visible and measurable steps to relieve the poor).
A proper Christian sense of *mystery*, then, doesn't take concepts such as "spiritual" and "practical," and immediately assume that they're opposed to one another. Rather, it sees them--along with everything else--as part of the entire mystery of creation and redemption. Thus, we lose that sense of "outrage" towards the evils of the world, since we realize that evil dwells in our own hearts as well. We're content to follow the path God has laid out for us, confident that we are doing our part to build up the Kingdom of God in this world.