Despite some valid points in comments on the question regarding assumptions, I think this is an interesting topic so I did some research and found a spectrum of articles and data. Let's parse this out a little...
Leanings of employees
First, let's focus on "any good research pointing to why so many organizations' workforces tend toward one extreme or the other, instead of representing a statistical mean of the population." It seems you're most interested employees rather than their boards and lobbying decisions.
Based on what I found, I'm not sure so many employees do tend towards one extreme or the other. I think we sometimes have the impression they do due to decisions made by the executives at a company but this might not actually represent the way their workforce's views. One obvious example is how many companies executives are anti-union but clearly their workers who are trying to unionize don't feel the same way.
Anyway, here are some actual articles and data...
- One way of measuring the way employees lean is by their donations. This, however can be skewed by more well paid employees who donate a lot.
- A more direct approach of measuring this would be surveys of employees but these can be flawed by factors like self selection, fear of retaliation and executives simply blocking the survey from being conducted in the first place.
- Another would potentially be extrapolating from external views of the company. You could narrow the data based on where employees are located and then try to account for some percentage of people avoiding the company due to their political views.
Clearly none of these approaches are perfect, but the second one seems like it would best answer your question. However while I'm sure there's been some surveys like this, I couldn't find any public ones.
In terms of the why aspect of the question here are some articles that attempt to explain it for tech, however I found very little data-driven research on it...
Leanings of executives and board members
There seems to be a lot more data and research on this group, maybe because there's more pressure on them -- especially recently -- to make their views public...
The high of a percentage of independents in some cases stand out as well as what the data seems to indicate about different leanings per gender.
Company lobbying and donations
Last but not least, and maybe the most plentiful in terms of actual data, you can find a lot about companies' lobbying spend and donations. This is likely because there are laws requiring that data to be public. Keep in mind though, this does not necessarily reflect each company or industry's employees' views...