The recent series of posts expressing indignation that many people who work for the USCCB lean left reminds me of a pet theory of mine: All other things being equal, people working for non-profits will tend to lean farther left than the general population.
This fits pretty well with my experience, both seeing most of my more progressive friends seek work at non-profits (in the cases of religious ones, often parish or diocesan work.) But I think there are some general reasons why we'd see this be the case.
1) Selection bias: It's one of the major themes of modern progressivism to be suspicious of the profit motive in general and of for-profit corporations in particular. If you see an organization making a profit as being particularly corrupting, it makes sense you'd gravitate towards organizations which are committed to provide a service to society without making a profit. You can see a reflection of this attitude in President Obama's proposal to forgive college debt for people who go into non-profit or government work -- behind which lies an implicit assumption that people working for non-profits and for the government are participating in work that is more virtuous or more valuable to society than people who work for mere businesses. (My impression is that conservatives tend more towards a "job is a job" attitude, seeing non-profit jobs as not being all that different from business jobs.)
2) Occupational influence: More subtly, I think that doing non-profit work often causes people to lean further left, at least in terms of being more in favor of taxation to fund a broader range of social programs. Why? The fact of the matter is that most people do not give very much money to help non-profits. There are, of course, a few non-profits who have wide appeal to folks with lots of money, and thus find themselves looking around for what to spend money on. But most non-profits find themselves scrounging for enough money to operate most of the time. I was rather shocked, when I became involved enough in our parish to get to see the finances of how a parish operates, to discover that if you have an average household income, and give 4-5% of it to your parish, you will be in the top 5-10% of donors. To a lot of people, supporting non-profits means buying from the occasional bake-sale and dropping a couple singles in the collection basket at Church on Sunday. It's fairly natural, I think, that people who routinely see what they believe is necessary and important work going undone because most people don't donate or donate very little, will come around to seeing the idea of taxation to support social programs as being a good idea. On the one hand, they have a very personal idea of the good which social programs might achieve (if it's your work that's wanting, you're likely to feel sure that if money were available, it would be well spent) and on the other they come in regular contact with the fact that most people don't voluntarily hand over much of their money. (That it is conservatives who donate the most to charity, and "moderates" who donate the least, is something which few people know and take into account.)
There is, perhaps, yet another factor, though since this is entirely anecdotal I advance it only hesitantly. Some years ago, I did web development work for a number of Catholic non-profits, and one of the things which struck me is how often people people would observe about some fairly minor screw-up, "Of course, in the business world heads would roll over this." My experience in the business world is that heads actually do very little rolling. If anything, the for-profit companies I've worked for were less likely to demand unpaid overtime, sink into vicious back-biting, or ream people out for minor errors. (This is perhaps an error in which the much scorned HR departments can be thanked.) But whatever the reason, there seemed to be a conviction that employment in the for-profit sector was nasty, brutish and short in the extreme. If such is a general impression among people working for non-profits, it probably adds further to the idea that for-profit enterprises provide virtually non benefit to society, and cause many harms.
While I think it is to the best if people in the non-profit and government sectors gain a better understanding of what business world and of free market economics, it is probably inevitable that these forces will cause many people working in the non-profit world to lean leftwards, and while it is a serious problem if this causes Catholic organizations to fund or support organizations which are supporting causes which are clearly wrong from a Catholic point of view (abortion, euthanasia, same sex marriage, distributing birth control, etc.) one can hardly be surprised at the tendency itself.